fix: Add framebuffer release/realloc and improved lazy indexing (#2563)

lib/hal/HalDisplay.cpp

@@ -77,6 +77,10 @@ void HalDisplay::deepSleep() { einkDisplay.deepSleep(); }
 
 uint8_t* HalDisplay::getFrameBuffer() const { return einkDisplay.getFrameBuffer(); }
 
+uint8_t* HalDisplay::lendFrameBufferStorage(uint32_t* sizeOut) { return einkDisplay.lendBuildStorage(sizeOut); }
+
+void HalDisplay::returnFrameBufferStorage() { einkDisplay.returnBuildStorage(); }
+
 void HalDisplay::copyGrayscaleBuffers(const uint8_t* lsbBuffer, const uint8_t* msbBuffer) {
   einkDisplay.copyGrayscaleBuffers(lsbBuffer, msbBuffer);
 }

lib/hal/HalDisplay.h

@@ -47,6 +47,14 @@ class HalDisplay {
   // Access to frame buffer
   uint8_t* getFrameBuffer() const;
 
+  // Lend the framebuffer's ~48 KB STORAGE to a memory-hungry phase (chapter
+  // builds) without freeing it: the allocation never moves, so repeated loans
+  // cannot fragment the heap (free+realloc measurably did). No display calls
+  // between lend and return; the panel keeps its last refreshed image. The
+  // buffer comes back white — redraw fully. Returns nullptr if already lent.
+  uint8_t* lendFrameBufferStorage(uint32_t* sizeOut);
+  void returnFrameBufferStorage();
+
   // 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