Initial display_partial() implementation

custom_components/waveshare_epaper_2bit/waveshare_epaper_2bit.h

@@ -448,6 +448,10 @@ class WaveshareEPaper2P9InV2R22Bpp : public WaveshareEPaper {
 
   void display() override;
 
+  // Fast 1-bit partial refresh. Non-blocking: skips if display is busy.
+  // Suitable for animation feedback; grayscale is restored on next display().
+  void display_partial();
+
   void fill(Color color) override;
 
   void dump_config() override;
@@ -471,6 +475,9 @@ class WaveshareEPaper2P9InV2R22Bpp : public WaveshareEPaper {
   uint32_t full_update_every_{30};
   uint32_t at_update_{0};
   uint8_t *buffer2_{nullptr};
+  // Tracks last-sent 1-bit state in standard convention (bit=1=white) for use
+  // as the 0x26 old-frame reference in partial updates.
+  uint8_t *old_buffer_bw_{nullptr};
 
  private:
   void reset_();