perf(sync): share the bounded converter cache for section percentages

Per-annotation percentage derivation (deriveAnnotationPercentage) built a
fresh CFIConverter for every highlight/note/bookmark, re-reading and
re-parsing the whole EPUB each time. Export SectionPercentageCached so
handlers reach the same bounded cache ConvertToCanonical already uses
(8 books, insertion-order eviction): one parse per book per push instead
of one per annotation.

internal/sync/locators.go

@@ -58,6 +58,13 @@ func cachedConverter(epubPath string) *CFIConverter {
 	return c
 }
 
+// SectionPercentageCached resolves the spine-section percentage of a CRE
+// xpointer through the shared bounded converter cache, so per-annotation
+// lookups parse the EPUB once per book instead of once per annotation.
+func SectionPercentageCached(epubPath, xpointer string) float64 {
+	return cachedConverter(epubPath).SectionPercentage(xpointer)
+}
+
 func ConvertToCanonical(
 	source LocatorSource,
 	devicePos string,