Add framebuffer lending for memory-intensive builds

Temporarily release framebuffer memory during section pagination to reduce heap pressure. The framebuffer can be released before memory-intensive operations and restored afterward, allowing BLE stack startup threshold to be lowered from 80KB to 70KB. Implements RAII-style FrameBufferBuildLoan class to ensure proper cleanup and automatic restart on restoration failure.

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 RAM to a memory-hungry phase. No display calls may
+  // run between release and a successful realloc; buffers come back white, so
+  // callers must redraw the full screen.
+  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