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.

lib/EpdFont/SdCardFont.h

@@ -55,6 +55,23 @@ class SdCardFont {
   // Returns the 12.4 fixed-point advance, or 0 if not found.
   uint16_t getAdvance(uint32_t codepoint, uint8_t style) const;
 
+  // Layout-side metric access — the per-glyph path CpFontAdapter uses while
+  // the FreeInkBook engine measures a chapter. Neither touches bitmap data:
+  // going through getGlyph() would stream every occurrence's bitmap through
+  // the 8-slot overflow ring (one SD read per character — a crawling build).
+  //
+  // hasGlyphMeta answers existence from the resident interval table (no I/O).
+  bool hasGlyphMeta(uint8_t styleIdx, uint32_t codepoint) const;
+  // ensureAdvance returns the 12.4 fixed-point advanceX, fetching the glyph
+  // METADATA from SD once on a miss and merging it into the persistent
+  // advance table (so each unique codepoint costs one small read per
+  // session). Returns 0 when the style lacks the codepoint.
+  uint16_t ensureAdvance(uint8_t styleIdx, uint32_t codepoint);
+
+  // Recover the style index from an EpdFontData::glyphMissCtx (see
+  // fromMissCtx below for recovering the SdCardFont itself).
+  static uint8_t styleIdxFromMissCtx(void* ctx);
+
   // Returns true if advance table is populated for at least one style.
   bool hasAdvanceTable() const;