Add framebuffer release and font metadata access

Introduce methods to temporarily release the 48KB framebuffer during memory-intensive chapter builds, allowing the memory to be reused. Add font metadata query methods (hasGlyphMeta, ensureAdvance) that check glyph existence and fetch advance metrics without loading full bitmap data, avoiding expensive SD reads during layout measurement.

src/activities/reader/BookPaginator.h

@@ -93,6 +93,11 @@ class BookPaginator {
   // Everything layout-relevant, hashed — the cache key.
   uint32_t generation() const;
 
+  // True when the chapter already has a page cache for the current
+  // generation — i.e. ensureChapter() will be a fast open, not a build.
+  // Callers use this to show the indexing popup before the heavy path.
+  bool isChapterCached(uint16_t spineIndex);
+
   // Opens the chapter's page cache for the current generation, laying the
   // chapter out first when missing or stale. Heavy only on that miss.
   freeink::book::BookStatus ensureChapter(uint16_t spineIndex, const BuildProgress& progress = BuildProgress());
@@ -131,10 +136,8 @@ class BookPaginator {
   // high-water). Deliberately snug: open needs scratch + the 48 KB temp book
   // arena simultaneously, and the C3's largest free block hovers just above
   // 112 KB — 64 KB here made the pair miss fitting by a few dozen bytes.
+  // (Chapter builds size their two arenas adaptively in ensureChapter.)
   static constexpr size_t kOpenScratchSize = 56 * 1024;
-  // Chapter pagination: layout buffers + inflate + writer index. The ideal
-  // size; ensureChapter steps down when fragmentation denies a block this big.
-  static constexpr size_t kBuildScratchSize = 120 * 1024;
 
   bool buildFontChain(GfxRenderer& renderer);
   bool reallocChapterArenas();