feat(server): progress locator coherence — percentage-only writes clear stale anchors

The verify/heal pipeline verified only INBOUND context-bearing writes;
stored-row coherence was never enforced. SaveProgress seeds missing
fields from the existing row, so every percentage-only write (KOReader
low-confidence conversion, KOReader no-pointer payloads, Kobo markup
ReadingSync / analytics / sync-from-server, locator-less queue entries)
silently re-paired its NEW percentage with the PREVIOUS writer's stale
CFI+context — the incoherent row shape that fed the client-side
far-anchor restore floor (and, via the read-side anchor handles, could
even hand a first-party client the OLD position's scroll address).

KOReader's converter already declared the right intent ("storing
percentage only — the row's percentage restores approximately"); the
seeding betrayed it. SaveProgressRequest gains ClearLocator, set by the
percentage-only call sites, and the seeding rule honors it: stale
epubcfi/context/character_offset clear (the upsert assigns columns
directly, so NULL sticks) unless the request carries its own. A
request-carried context survives so the verify/heal pass can still mint
a fresh anchor from it; the HTTP merge path (field-absent PUTs
preserve) and all anchor-bearing writes are untouched. The fixed-layout
branch already cleared locators unconditionally — unchanged.

Consumer audit: percentage — the only thing every sync round-trip
exchanges — is untouched on all paths; KOReader pulls on locator-less
rows already support the nil-CFI shape (builder guards
progress.Epubcfi.Valid); first-party restores go percentage-first
instead of floor-fighting a stale anchor.

Tests: TestProgressWeb_LocatorCoherence pins clear-on-declared,
request-carried-context survival, and merge-path preservation. Full
suite with the change: 218 groups, zero failures.

cmd/server/tests/progress_service_test.go

@@ -656,3 +656,117 @@ func TestProgressWeb_EdgeCases(t *testing.T) {
 		assert.InDelta(t, 1.5, progress.ZoomLevel.Float64, 0.01)
 	})
 }
+
+// TestProgressWeb_LocatorCoherence pins the locator coherence rule: a
+// percentage-only write that DECLARES no locator (ClearLocator) must clear
+// any inherited stale anchor instead of re-pairing the new percentage with
+// the previous writer's CFI/context — the incoherent row shape that fed the
+// client-side far-anchor restore floor. Request-carried fields always win.
+func TestProgressWeb_LocatorCoherence(t *testing.T) {
+	setup := setupTestServer(t)
+	ctx := context.Background()
+	userID := getTestUserID(t, setup.DB)
+	pgUserID := pgtype.UUID{Bytes: userID, Valid: true}
+
+	seedAnchor := func(t *testing.T, mediaItemID string) uuid.UUID {
+		mediaUUID, _ := uuid.Parse(mediaItemID)
+		_, err := setup.ProgressService.SaveProgress(ctx, wsync.SaveProgressRequest{
+			MediaItemID: pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:      pgUserID,
+			Source:      "web",
+			Percentage:  pFloat64(0.3),
+			Epubcfi:     pStr("epubcfi(/6/4/2:first)"),
+			ContextText: pStr("some anchor context sentence"),
+			DeviceType:  "web",
+			Broadcast:   false,
+		})
+		require.NoError(t, err)
+		progress, err := setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{
+			MediaItemID: pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:      pgUserID,
+		})
+		require.NoError(t, err)
+		require.True(t, progress.Epubcfi.Valid, "seed: anchor stored")
+		return mediaUUID
+	}
+
+	t.Run("percentage-only kobo write clears the inherited anchor", func(t *testing.T) {
+		mediaItemID := createTestMediaItemID(t, setup)
+		mediaUUID := seedAnchor(t, mediaItemID)
+
+		_, err := setup.ProgressService.SaveProgress(ctx, wsync.SaveProgressRequest{
+			MediaItemID:  pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:       pgUserID,
+			Source:       "kobo",
+			Percentage:   pFloat64(0.6),
+			DeviceType:   "kobo",
+			ClearLocator: true,
+			Broadcast:    false,
+		})
+		require.NoError(t, err)
+
+		progress, err := setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{
+			MediaItemID: pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:      pgUserID,
+		})
+		require.NoError(t, err)
+		assert.InDelta(t, 0.6, progress.Percentage.Float64, 0.001, "percentage updated")
+		assert.False(t, progress.Epubcfi.Valid, "stale anchor cleared, not re-paired")
+		assert.False(t, progress.ContextText.Valid, "stale context cleared")
+		assert.Equal(t, "kobo", progress.LastSyncSource.String)
+	})
+
+	t.Run("request-carried context survives the clear", func(t *testing.T) {
+		mediaItemID := createTestMediaItemID(t, setup)
+		mediaUUID := seedAnchor(t, mediaItemID)
+
+		_, err := setup.ProgressService.SaveProgress(ctx, wsync.SaveProgressRequest{
+			MediaItemID:  pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:       pgUserID,
+			Source:       "koreader",
+			Percentage:   pFloat64(0.7),
+			ContextText:  pStr("a brand new context sentence"),
+			DeviceType:   "koreader",
+			ClearLocator: true,
+			Broadcast:    false,
+		})
+		require.NoError(t, err)
+
+		progress, err := setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{
+			MediaItemID: pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:      pgUserID,
+		})
+		require.NoError(t, err)
+		assert.InDelta(t, 0.7, progress.Percentage.Float64, 0.001)
+		assert.True(t, progress.ContextText.Valid, "request-carried context kept")
+		assert.Equal(t, "a brand new context sentence", progress.ContextText.String)
+		// The verify gate is inert in the test harness (no media path
+		// resolver wired), so no anchor can be minted here; the rule
+		// under test is that the STALE one is gone.
+		assert.False(t, progress.Epubcfi.Valid, "stale anchor cleared")
+	})
+
+	t.Run("HTTP merge semantics untouched: key-absent PUT preserves fields", func(t *testing.T) {
+		mediaItemID := createTestMediaItemID(t, setup)
+		mediaUUID := seedAnchor(t, mediaItemID)
+
+		// Service-level save WITHOUT ClearLocator mimics the HTTP merge
+		// path (field absent = inherit): the anchor must survive.
+		_, err := setup.ProgressService.SaveProgress(ctx, wsync.SaveProgressRequest{
+			MediaItemID: pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:      pgUserID,
+			Source:      "queue",
+			Percentage:  pFloat64(0.4),
+			Broadcast:   false,
+		})
+		require.NoError(t, err)
+
+		progress, err := setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{
+			MediaItemID: pgtype.UUID{Bytes: mediaUUID, Valid: true},
+			UserID:      pgUserID,
+		})
+		require.NoError(t, err)
+		assert.True(t, progress.Epubcfi.Valid, "merge path preserves the anchor")
+		assert.Equal(t, "epubcfi(/6/4/2:first)", progress.Epubcfi.String)
+	})
+}

internal/handlers/kobo.go

@@ -458,6 +458,7 @@ func (h *KoboHandler) Markup(c *echo.Context) error {
 				DeviceType:   "kobo",
 				DeviceName:   device.DeviceName,
 				Broadcast:    true,
+				ClearLocator: true,
 			})
 		} else {
 			_, err = h.db.UpdateUniversalProgress(c.Request().Context(), database.UpdateUniversalProgressParams{
@@ -806,6 +807,7 @@ func (h *KoboHandler) AnalyticsGettests(c *echo.Context) error {
 				DeviceType:   "kobo",
 				DeviceName:   device.DeviceName,
 				Broadcast:    true,
+				ClearLocator: true,
 			})
 		} else {
 			_, err = h.db.UpdateUniversalProgress(c.Request().Context(), database.UpdateUniversalProgressParams{
@@ -868,6 +870,7 @@ func (h *KoboHandler) SyncFromServer(c *echo.Context) error {
 				DeviceType:   "kobo",
 				DeviceName:   device.DeviceName,
 				Broadcast:    false,
+				ClearLocator: true,
 			})
 		} else {
 			_, err = h.db.UpdateUniversalProgress(c.Request().Context(), database.UpdateUniversalProgressParams{

internal/handlers/koreader.go

@@ -862,6 +862,11 @@ func (h *KOReaderHandler) updateProgressForBook(c *echo.Context, deviceID pgtype
 
 	if h.progressSvc != nil {
 		epubcfi := book.Epubcfi
+		// No position pointer from the device, or a conversion that
+		// declined below: the write is percentage-only by intent. The
+		// service then clears any inherited stale anchor instead of
+		// re-pairing it with the new percentage.
+		clearLocator := epubcfi == nil
 		if epubcfi != nil && wsync.IsCREXPointer(*epubcfi) {
 			// Same facade every annotation uses: cached converter,
 			// structural-first resolution, guarded fallbacks. The facade
@@ -897,6 +902,7 @@ func (h *KOReaderHandler) updateProgressForBook(c *echo.Context, deviceID pgtype
 					// and the device keeps its own native position.
 					log.Printf("Bookhoard: CRE→CFI conversion low-confidence (%s) for %s; storing percentage only", loc.Precision, *epubcfi)
 					epubcfi = nil
+					clearLocator = true
 				}
 			}
 		}
@@ -916,6 +922,7 @@ func (h *KOReaderHandler) updateProgressForBook(c *echo.Context, deviceID pgtype
 			DeviceType:      "koreader",
 			DeviceName:      deviceModel,
 			Broadcast:       true,
+			ClearLocator:    clearLocator,
 		}
 
 		_, err := h.progressSvc.SaveProgress(ctx, saveReq)

internal/sync/progress.go

@@ -315,6 +315,16 @@ type SaveProgressRequest struct {
 	DeviceType string
 	DeviceName string
 	Broadcast  bool
+
+	// ClearLocator declares that this write INTENDS to carry no position
+	// locator: percentage-only writers (devices whose converter declined,
+	// Kobo markup percentage syncs, queue entries without CFI). Without it,
+	// seeding below would re-pair the new percentage with the PREVIOUS
+	// writer's stale CFI/context — the incoherent row shape that feeds the
+	// client-side far-anchor restore floor. Locater-less rows are honest:
+	// percentage restores are approximate, and the next anchor-bearing
+	// write re-establishes the pair. Request-supplied fields always win.
+	ClearLocator bool
 }
 
 func (s *ProgressService) SaveProgress(ctx context.Context, req SaveProgressRequest) (database.ReadingProgress, error) {
@@ -407,6 +417,22 @@ func (s *ProgressService) SaveProgress(ctx context.Context, req SaveProgressRequ
 		params.ReadingMode = pgtype.Text{String: *req.ReadingMode, Valid: true}
 	}
 
+	// Locator coherence: a write that declares no locator must not
+	// silently re-pair its new percentage with the previous writer's
+	// stale CFI/context (inherited by the seeding above). Clear them so
+	// the row is an honest percentage-only position. Request-carried
+	// fields always win — and a request-carried context survives so the
+	// verify/heal pass below can mint a fresh anchor from it.
+	if req.ClearLocator && req.Epubcfi == nil {
+		params.Epubcfi = pgtype.Text{}
+		if req.ContextText == nil {
+			params.ContextText = pgtype.Text{}
+		}
+		if req.CharacterOffset == nil {
+			params.CharacterOffset = pgtype.Int8{}
+		}
+	}
+
 	params.LastSyncDevice = pgtype.Text{String: req.Source, Valid: true}
 	params.LastSyncSource = pgtype.Text{String: req.Source, Valid: true}
 

internal/sync/queue.go

@@ -443,6 +443,10 @@ func (p *SyncQueueProcessor) syncProgress(ctx context.Context, userID pgtype.UUI
 		if v, ok := syncData["epubcfi"].(string); ok {
 			req.Epubcfi = &v
 		}
+		// Entries without a CFI are percentage-only by intent: declare it
+		// so the service clears any inherited stale anchor instead of
+		// re-pairing it with this percentage.
+		req.ClearLocator = req.Epubcfi == nil
 		if v, ok := syncData["context_text"].(string); ok {
 			req.ContextText = &v
 		}