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.

internal/sync/queue.go

@@ -126,29 +126,29 @@ func (p *SyncQueueProcessor) EnqueueProgress(update *ProgressUpdate) error {
 }
 
 type HighlightUpdate struct {
-	DeviceID       pgtype.UUID
-	MediaItemID    pgtype.UUID
-	UserID         pgtype.UUID
-	SelectionText  string
-	StartPosition  string
-	EndPosition    string
-	Color          string
-	NoteText       string
-	EpubcfiStart   string
-	EpubcfiEnd     string
+	DeviceID        pgtype.UUID
+	MediaItemID     pgtype.UUID
+	UserID          pgtype.UUID
+	SelectionText   string
+	StartPosition   string
+	EndPosition     string
+	Color           string
+	NoteText        string
+	EpubcfiStart    string
+	EpubcfiEnd      string
 	PercentageStart float64
 	PercentageEnd   float64
-	Source         string
-	DeviceSyncData map[string]interface{}
+	Source          string
+	DeviceSyncData  map[string]interface{}
 }
 
 type NoteUpdate struct {
-	DeviceID     pgtype.UUID
-	MediaItemID  pgtype.UUID
-	UserID       pgtype.UUID
-	Content      string
-	Position     string
-	Source       string
+	DeviceID       pgtype.UUID
+	MediaItemID    pgtype.UUID
+	UserID         pgtype.UUID
+	Content        string
+	Position       string
+	Source         string
 	DeviceSyncData map[string]interface{}
 }
 
@@ -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
 		}