perf(sync): share parsed EPUBs across conversions, make converters concurrency-safe

ConvertToCanonical/ConvertFromCanonical built a fresh CFIConverter
per call, and each annotation converts twice (pos0+pos1) — a book
with 200 highlights re-opened and re-parsed the EPUB 400+ times per
sync, and again per metadata pull. A bounded 8-entry cache keyed by
path now shares converters (the parsing work belongs on the server;
clients stay thin). CFIConverter gained a mutex around its lazily
built spine/doc caches since instances are now shared between
concurrent requests.

Adds CFIConverter.SectionPercentage: book-wide percentage for a CRE
xpointer from the spine char distribution (midpoint of its document)
— the server-side counterpart to dropping per-annotation
getPageFromXPointer lookups from the plugin.

internal/sync/locators.go

@@ -1,6 +1,9 @@
 package sync
 
-import "log"
+import (
+	"log"
+	"sync"
+)
 
 type LocatorSource string
 
@@ -26,6 +29,35 @@ func isConvertible(formatGroup string) bool {
 	return formatGroup == string(FormatGroupReflowable)
 }
 
+// Converters parse and cache the whole EPUB (spine + content docs), so
+// creating one per annotation re-reads the book for every entry. A small
+// bounded cache lets one request — or several — share a single parse.
+// Servers are the right place for this work: clients stay thin.
+var (
+	converterMu    sync.Mutex
+	converterCache = map[string]*CFIConverter{}
+	converterOrder []string // insertion order for eviction
+)
+
+const maxCachedConverters = 8
+
+func cachedConverter(epubPath string) *CFIConverter {
+	converterMu.Lock()
+	defer converterMu.Unlock()
+	if c, ok := converterCache[epubPath]; ok {
+		return c
+	}
+	c := NewCFIConverter(epubPath)
+	converterCache[epubPath] = c
+	converterOrder = append(converterOrder, epubPath)
+	for len(converterOrder) > maxCachedConverters {
+		oldest := converterOrder[0]
+		converterOrder = converterOrder[1:]
+		delete(converterCache, oldest)
+	}
+	return c
+}
+
 func ConvertToCanonical(
 	source LocatorSource,
 	devicePos string,
@@ -48,7 +80,7 @@ func ConvertToCanonical(
 		if !IsCREXPointer(devicePos) {
 			return CanonicalLocator{CFI: devicePos, Precision: "already-standard", Percentage: percentage}
 		}
-		converter := NewCFIConverter(epubPath)
+		converter := cachedConverter(epubPath)
 		result, err := converter.ConvertCREToStandard(devicePos, percentage, contextText)
 		if err != nil || result == nil {
 			log.Printf("Bookhoard: locator CRE→CFI conversion failed: %v", err)
@@ -101,7 +133,7 @@ func ConvertFromCanonical(
 
 	switch source {
 	case LocatorSourceKOReader:
-		converter := NewCFIConverter(epubPath)
+		converter := cachedConverter(epubPath)
 		result, err := converter.ConvertStandardToCRE(canonicalCFI, percentage, contextText)
 		if err != nil || result == nil {
 			log.Printf("Bookhoard: locator CFI→CRE conversion failed: %v", err)