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.cpp

@@ -1061,6 +1061,28 @@ void SdCardFont::mergeIntoAdvanceTable(uint8_t styleIdx, const AdvanceEntry* sor
   advanceTableSize_[styleIdx] = k;
 }
 
+bool SdCardFont::hasGlyphMeta(uint8_t styleIdx, uint32_t codepoint) const {
+  styleIdx &= (MAX_STYLES - 1);
+  const PerStyle& s = styles_[styleIdx];
+  if (!loaded_ || !s.present) return false;
+  return findGlobalGlyphIndex(s, codepoint) >= 0;
+}
+
+uint16_t SdCardFont::ensureAdvance(uint8_t styleIdx, uint32_t codepoint) {
+  styleIdx &= (MAX_STYLES - 1);
+  if (!loaded_ || !styles_[styleIdx].present) return 0;
+  const uint16_t cached = getAdvance(codepoint, styleIdx);
+  if (cached != 0) return cached;
+  // 0 is either "not in table" or a genuine zero-width glyph; only pay the
+  // SD read when the resident interval table says the glyph exists.
+  if (findGlobalGlyphIndex(styles_[styleIdx], codepoint) < 0) return 0;
+  uint32_t cp = codepoint;
+  fetchAdvancesForCodepoints(&cp, 1, static_cast<uint8_t>(1u << styleIdx));
+  return getAdvance(codepoint, styleIdx);
+}
+
+uint8_t SdCardFont::styleIdxFromMissCtx(void* ctx) { return static_cast<OverflowContext*>(ctx)->styleIdx; }
+
 bool SdCardFont::hasAdvanceTable() const {
   for (uint8_t i = 0; i < MAX_STYLES; i++) {
     if (advanceTable_[i]) return true;