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);
 }