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_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")
+		}
+	})
+}