feat(screenshot): extend e-ink screenshot support via GxEPD2 patch

- Add lib/GxEPD2-patch/src/GxEPD2_BW.h: local patched copy of GxEPD2_BW.h
  that exposes getBuffer()/getBufferSize() under ENABLE_SCREENSHOT guard.
  Include guard (_GxEPD2_BW_H_) prevents double-inclusion of the installed
  library version.

- GxEPDDisplay.h: use patched header via relative include when
  ENABLE_SCREENSHOT so the patch takes precedence over the installed lib
  (PlatformIO adds library paths before -I build_flags).

- DisplayDriver.h: add virtual getBuffer()/getBufferSize()/getDisplayType()
  defaults (nullptr/0/0) under ENABLE_SCREENSHOT.

- SH1106Display.h / SSD1306Display.h / GxEPDDisplay.h: add concrete overrides;
  getDisplayType() returns 0 (OLED) or 1 (e-ink).

- MyMesh.cpp/h: replace fragile C-cast with virtual dispatch in
  handleScreenshotRequest(); extend 5-byte header to 6 bytes by appending
  display_type so the host tool can decode the correct pixel layout.

- tools/screenshot.py: parse 6-byte header; add eink_buffer_to_image()
  that decodes the row-major MSB-first GxEPD2 buffer with DISPLAY_ROTATION=1
  (phys_x = 127-ly, phys_y = lx); dispatch on display_type.

- variants/wio-tracker-l1-eink/platformio.ini: add
  [env:WioTrackerL1Eink_companion_dual_dev] with ENABLE_SCREENSHOT.
- variants/wio-tracker-l1/platformio.ini: unchanged (OLED env already exists).

Builds verified: WioTrackerL1Eink_companion_dual_dev SUCCESS,
                 WioTrackerL1Eink_companion_radio_ble SUCCESS (unaffected).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/ui/DisplayDriver.h

@@ -247,4 +247,12 @@ public:
   virtual void setDisplayRotation(uint8_t rot) { }  // 0-3, no-op for fixed-orientation displays
   virtual void setFullRefreshInterval(uint8_t n) { }  // e-ink: do full refresh every n partial refreshes (0=never)
   virtual void endFrame() = 0;
+
+#ifdef ENABLE_SCREENSHOT
+  // Screenshot support — return raw framebuffer and its size in bytes.
+  // 0=OLED (page-based, column-major), 1=e-ink (row-major, MSB-first, 1=white/0=black).
+  virtual const uint8_t* getBuffer() { return nullptr; }
+  virtual uint16_t getBufferSize() { return 0; }
+  virtual uint8_t getDisplayType() { return 0; }
+#endif
 };

src/helpers/ui/GxEPDDisplay.h

@@ -5,7 +5,15 @@
 
 #define ENABLE_GxEPD2_GFX 0
 
+// When ENABLE_SCREENSHOT is active, use the patched copy of GxEPD2_BW.h from
+// lib/GxEPD2-patch/src/ that exposes getBuffer()/getBufferSize().  The include
+// guard (_GxEPD2_BW_H_) then prevents the installed library version from
+// being compiled again.  Non-screenshot builds use the installed library as-is.
+#ifdef ENABLE_SCREENSHOT
+#include "../../../lib/GxEPD2-patch/src/GxEPD2_BW.h"
+#else
 #include <GxEPD2_BW.h>
+#endif
 #include <GxEPD2_3C.h>
 #include <GxEPD2_4C.h>
 #include <GxEPD2_7C.h>
@@ -70,6 +78,12 @@ public:
     display(GxEPD2_150_BN(DISP_CS, DISP_DC, DISP_RST, DISP_BUSY)) {}
 #endif
 
+#ifdef ENABLE_SCREENSHOT
+  const uint8_t* getBuffer() override { return display.getBuffer(); }
+  uint16_t getBufferSize() override { return display.getBufferSize(); }
+  uint8_t getDisplayType() override { return 1; }  // 1 = e-ink
+#endif
+
   // Line height and approx. char width for each font size:
   //   1 = FreeSans9pt      (lineH=16, charW≈9)
   //   2 = FreeSansBold12pt (lineH=20, charW≈12)

src/helpers/ui/SH1106Display.h

@@ -60,7 +60,8 @@ public:
   void endFrame() override;
   
 #ifdef ENABLE_SCREENSHOT
-  // Screenshot support
-  uint8_t* getBuffer() { return display.getBuffer(); }
+  const uint8_t* getBuffer() override { return display.getBuffer(); }
+  uint16_t getBufferSize() override { return (uint16_t)((width() * height()) / 8); }
+  uint8_t getDisplayType() override { return 0; }
 #endif
 };

src/helpers/ui/SSD1306Display.h

@@ -47,7 +47,8 @@ public:
   void endFrame() override;
   
 #ifdef ENABLE_SCREENSHOT
-  // Screenshot support
-  uint8_t* getBuffer() { return display.getBuffer(); }
+  const uint8_t* getBuffer() override { return display.getBuffer(); }
+  uint16_t getBufferSize() override { return (uint16_t)((width() * height()) / 8); }
+  uint8_t getDisplayType() override { return 0; }
 #endif
 };