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/hal/HalDisplay.cpp

@@ -77,6 +77,10 @@ void HalDisplay::deepSleep() { einkDisplay.deepSleep(); }
 
 uint8_t* HalDisplay::getFrameBuffer() const { return einkDisplay.getFrameBuffer(); }
 
+void HalDisplay::releaseFrameBuffers() { einkDisplay.releaseBuffers(); }
+
+bool HalDisplay::reallocFrameBuffers() { return einkDisplay.reallocBuffers(); }
+
 void HalDisplay::copyGrayscaleBuffers(const uint8_t* lsbBuffer, const uint8_t* msbBuffer) {
   einkDisplay.copyGrayscaleBuffers(lsbBuffer, msbBuffer);
 }

lib/hal/HalDisplay.h

@@ -47,6 +47,12 @@ class HalDisplay {
   // Access to frame buffer
   uint8_t* getFrameBuffer() const;
 
+  // Lend the framebuffer's ~48 KB to a memory-hungry phase (chapter builds).
+  // No display calls between release and a successful realloc; the panel
+  // keeps its last refreshed image. Buffers come back white — redraw fully.
+  void releaseFrameBuffers();
+  bool reallocFrameBuffers();
+
   // X3 grayscale preconditioning (OEM "AA-pre-BW(mid)" settle pass), windowed
   // to the gray region in physical panel coordinates (no-arg = full frame).
   // Call after the BW base frame is displayed and before the grayscale planes