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

lib/GfxRenderer/GfxRenderer.h

@@ -250,6 +250,34 @@ class GfxRenderer {
   // Font helpers
   const uint8_t* getGlyphBitmap(const EpdFontData* fontData, const EpdGlyph* glyph) const;
 
+  // Lend the 48 KB framebuffer's bytes to a memory-hungry phase (chapter
+  // builds) WITHOUT freeing the allocation, so it never moves and repeated
+  // loans cannot fragment the heap. Between release and restore NOTHING may
+  // draw or display — the panel keeps showing its last refreshed image. The
+  // lent bytes are published via buildscratch::claim() for consumers like
+  // InflateStream. restore returns the buffer white, so the caller must
+  // redraw the full screen; it cannot fail (no allocation involved).
+  void releaseFrameBufferForBuild();
+  bool restoreFrameBufferAfterBuild();
+  bool hasFrameBuffer() const { return frameBuffer != nullptr; }
+
+  // RAII form of the loan above, for blocking build regions with early-return
+  // error paths: restores on scope exit (or explicitly via end()). Display the
+  // popup/screen the panel should hold BEFORE constructing one. Constructing
+  // while the framebuffer is already lent yields an inert loan (nesting-safe).
+  class FrameBufferLoan {
+   public:
+    explicit FrameBufferLoan(GfxRenderer& renderer);
+    ~FrameBufferLoan() { end(); }
+    void end();
+    FrameBufferLoan(const FrameBufferLoan&) = delete;
+    FrameBufferLoan& operator=(const FrameBufferLoan&) = delete;
+
+   private:
+    GfxRenderer& renderer_;
+    bool active_ = false;
+  };
+
   // Low level functions
   uint8_t* getFrameBuffer() const;
   size_t getBufferSize() const;