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/GfxRenderer/GfxRenderer.cpp

@@ -91,6 +91,20 @@ void GfxRenderer::begin() {
   bwBufferChunks.assign((frameBufferSize + BW_BUFFER_CHUNK_SIZE - 1) / BW_BUFFER_CHUNK_SIZE, nullptr);
 }
 
+void GfxRenderer::releaseFrameBufferForBuild() {
+  display.releaseFrameBuffers();
+  frameBuffer = nullptr;
+}
+
+bool GfxRenderer::restoreFrameBufferAfterBuild() {
+  if (!display.reallocFrameBuffers()) {
+    LOG_ERR("GFX", "Framebuffer realloc failed after build");
+    return false;
+  }
+  frameBuffer = display.getFrameBuffer();
+  return frameBuffer != nullptr;
+}
+
 bool GfxRenderer::isFontCacheScanning() const { return fontCacheManager_ && fontCacheManager_->isScanning(); }
 
 void GfxRenderer::insertFont(const int fontId, EpdFontFamily font) {

lib/GfxRenderer/GfxRenderer.h

@@ -253,6 +253,15 @@ class GfxRenderer {
   // Font helpers
   const uint8_t* getGlyphBitmap(const EpdFontData* fontData, const EpdGlyph* glyph) const;
 
+  // Lend the 48 KB framebuffer to a memory-hungry phase (chapter builds).
+  // Between release and a successful restore NOTHING may draw or display —
+  // the panel keeps showing its last refreshed image. restore returns the
+  // buffer white, so the caller must redraw the full screen; false means the
+  // heap could not re-supply the buffer (callers treat that as fatal).
+  void releaseFrameBufferForBuild();
+  bool restoreFrameBufferAfterBuild();
+  bool hasFrameBuffer() const { return frameBuffer != nullptr; }
+
   // Low level functions
   uint8_t* getFrameBuffer() const;
   size_t getBufferSize() const;