fix(sync): let content-equal echoes refresh drifted locators

The LWW skip path compared only annotation content (text, color, note,
percentages) — locator columns were not part of 'changed'. A device echo
with identical content therefore resolved to skip, and the freshly
re-derived canonical CFIs were discarded in the same request that
computed them: a highlight whose stored start anchor had been corrupted
by the old percentage-exact bug could never heal, because every
subsequent echo carried the same text and was skipped before the
locator columns were written. Observed live: a push converted the
cross-block highlight's anchors correctly (structural, heading to
paragraph) yet the row kept its garbage start CFI and the highlight
stayed unpaintable on the web.

Echo saves now treat a non-empty incoming locator that differs from the
stored one as a change (locatorRefresher): empty locators still coalesce
(no drift), and once healed the echo produces identical CFIs, so the
steady state remains skip — no write churn. Highlights check
epubcfi_start/end; bookmarks check cfi_position/position.

applyBookmarkLWW also gains the empty-locator coalescing the highlight
path already had: web bookmark edits carry no device locators, and a
title/note edit must not wipe the stored device-native position.

internal/sync/annotations.go

@@ -279,7 +279,9 @@ func (s *AnnotationService) compareIncoming(req SaveHighlightRequest, existing d
 			textEq(req.Color, existing.Color) &&
 			textEq(req.NoteText, existing.NoteText) &&
 			floatEq(req.PercentageStart, existing.PercentageStart) &&
-			floatEq(req.PercentageEnd, existing.PercentageEnd)
+			floatEq(req.PercentageEnd, existing.PercentageEnd) &&
+			locatorRefresher(req.EpubcfiStart, existing.EpubcfiStart) &&
+			locatorRefresher(req.EpubcfiEnd, existing.EpubcfiEnd)
 		return !contentSame, !contentSame
 	}
 
@@ -290,6 +292,22 @@ func (s *AnnotationService) compareIncoming(req SaveHighlightRequest, existing d
 	return req.ModifiedAt.After(existingMod.Time), true
 }
 
+// locatorRefresher reports whether an incoming locator leaves the stored
+// one untouched: either the push carried none (empty coalesces to the
+// stored value in applyLWW) or it matches what is stored. A non-empty
+// incoming locator that DIFFERS is a deliberate refresh: device echoes
+// re-derive their canonical CFIs on every push, and an improvement (a
+// converter fix re-landing a corrupted anchor, drift repair) must reach
+// the row even when the annotation content is otherwise identical —
+// without this, content-equal echoes resolve to "skip" and a bad stored
+// locator can never heal.
+func locatorRefresher(incoming string, existing pgtype.Text) bool {
+	if incoming == "" {
+		return true
+	}
+	return textEq(incoming, existing)
+}
+
 func (s *AnnotationService) TombstoneHighlight(
 	ctx context.Context,
 	userID, mediaItemID pgtype.UUID,
@@ -744,13 +762,25 @@ func (s *AnnotationService) applyBookmarkLWW(ctx context.Context, req SaveBookma
 	}
 	deviceData := mergeDeviceSyncData(existing.DeviceSyncData, req.Source, req.DeviceSyncData)
 
+	// Web saves carry no device locators: keep the stored ones so a web
+	// title/note edit never wipes the device-native position (mirrors the
+	// highlight path's coalescing).
+	cfiPosition := req.CFIPosition
+	if cfiPosition == "" {
+		cfiPosition = existing.CfiPosition.String
+	}
+	position := req.Position
+	if position == "" {
+		position = existing.Position.String
+	}
+
 	bm, err := s.db.UpdateMediaBookmarkForSync(ctx, database.UpdateMediaBookmarkForSyncParams{
 		ID:                 existing.ID,
 		PageNumber:         pgInt4(req.PageNumber),
 		ChapterNumber:      pgInt4(req.ChapterNumber),
-		CfiPosition:        pgText(req.CFIPosition),
+		CfiPosition:        pgText(cfiPosition),
 		Title:              req.Title,
-		Position:           pgText(req.Position),
+		Position:           pgText(position),
 		Notes:              pgText(req.Notes),
 		PercentageLocation: pgFloat8(req.PercentageLoc),
 		EpubcfiLocation:    pgText(req.EpubcfiLocation),
@@ -775,7 +805,9 @@ func (s *AnnotationService) applyBookmarkLWW(ctx context.Context, req SaveBookma
 func (s *AnnotationService) compareIncomingBookmark(req SaveBookmarkRequest, existing database.MediaBookmarks) (incomingNewer bool, contentChanged bool) {
 	if req.ModifiedAt.IsZero() {
 		contentSame := strings.EqualFold(req.Title, existing.Title) &&
-			textEq(req.Notes, existing.Notes)
+			textEq(req.Notes, existing.Notes) &&
+			locatorRefresher(req.CFIPosition, existing.CfiPosition) &&
+			locatorRefresher(req.Position, existing.Position)
 		return !contentSame, !contentSame
 	}
 	existingMod := existing.LastModifiedAt

internal/sync/annotations_test.go

@@ -369,3 +369,82 @@ func TestIncomingNewerThanTombstone(t *testing.T) {
 		})
 	}
 }
+
+// Content-equal device echoes must still count as changed when they carry
+// a locator that differs from the stored one: echoes re-derive canonical
+// CFIs on every push, and a better conversion (or a repair of a corrupted
+// anchor) has to reach the row — otherwise the skip path discards it and
+// the bad locator can never heal.
+func TestCompareIncomingLocatorDriftRefreshes(t *testing.T) {
+	svc := &AnnotationService{}
+	existing := database.MediaHighlights{
+		SelectionText: "CHAPTER IV. “What a pity it is, Elinor,”",
+		Color:         pgtype.Text{String: "#90caf9", Valid: true},
+		EpubcfiStart:  pgtype.Text{String: "epubcfi(/6/12!/4/2[x]/32/3:576)", Valid: true},
+		EpubcfiEnd:    pgtype.Text{String: "epubcfi(/6/12!/4/2[x]/4/1:93)", Valid: true},
+	}
+	base := SaveHighlightRequest{
+		SelectionText: existing.SelectionText,
+		Color:         "#90caf9",
+	}
+
+	t.Run("identical content and locators skip", func(t *testing.T) {
+		req := base
+		req.EpubcfiStart = existing.EpubcfiStart.String
+		req.EpubcfiEnd = existing.EpubcfiEnd.String
+		_, changed := svc.compareIncoming(req, existing)
+		if changed {
+			t.Error("identical echo must not rewrite the row")
+		}
+	})
+
+	t.Run("better start locator refreshes despite identical content", func(t *testing.T) {
+		req := base
+		req.EpubcfiStart = "epubcfi(/6/12!/4/2[x]/2/3:0)"
+		req.EpubcfiEnd = existing.EpubcfiEnd.String
+		_, changed := svc.compareIncoming(req, existing)
+		if !changed {
+			t.Error("locator drift on a content-equal echo must trigger an update")
+		}
+	})
+
+	t.Run("empty incoming locators leave the row alone", func(t *testing.T) {
+		req := base // no CFIs: applyLWW coalesces empty to stored
+		_, changed := svc.compareIncoming(req, existing)
+		if changed {
+			t.Error("empty locators coalesce; must not count as drift")
+		}
+	})
+}
+
+// Bookmarks share the echo-refresh rule for their locators.
+func TestCompareIncomingBookmarkLocatorDrift(t *testing.T) {
+	svc := &AnnotationService{}
+	existing := database.MediaBookmarks{
+		Title:       "in CHAPTER IV.",
+		CfiPosition: pgtype.Text{String: "epubcfi(/6/12!/4/2[x]/2/3:0)", Valid: true},
+		Position:    pgtype.Text{String: "/body/DocFragment[6]/body/div[1]/h2[1]/text().0", Valid: true},
+	}
+
+	t.Run("same locators skip", func(t *testing.T) {
+		req := SaveBookmarkRequest{
+			Title:       existing.Title,
+			CFIPosition: existing.CfiPosition.String,
+			Position:    existing.Position.String,
+		}
+		if _, changed := svc.compareIncomingBookmark(req, existing); changed {
+			t.Error("identical bookmark echo must not rewrite the row")
+		}
+	})
+
+	t.Run("new CFI refreshes despite same title", func(t *testing.T) {
+		req := SaveBookmarkRequest{
+			Title:       existing.Title,
+			CFIPosition: "epubcfi(/6/12!/4/2[x]/2/3:5)",
+			Position:    existing.Position.String,
+		}
+		if _, changed := svc.compareIncomingBookmark(req, existing); !changed {
+			t.Error("bookmark locator drift must trigger an update")
+		}
+	})
+}