feat(boards): Add Waveshare ESP32-C6-Touch-AMOLED-2.16 support

- Add complete board support for Waveshare ESP32-C6-Touch-AMOLED-2.16 with SH8601 display driver
- Implement board initialization with AXP2101 PMU rail configuration for LCD and touch power
- Add display driver for SH8601 QSPI panel (480×480 resolution)
- Add CST9217 touch controller support via I2C
- Add QMI8658 IMU initialization and sensor reading
- Add AXP2101 power management and battery monitoring
- Add input handling for BOOT button (GPIO 9)
- Configure PlatformIO environment with C6-specific build flags and 16 MB flash layout
- Update capability flags documentation to clarify BOARD_HAS_PSRAM build-flag macro usage
- C6 has no external PSRAM; shared code gates on this flag to use internal SRAM and reduce LVGL buffer sizes
- Screenshot capture disabled on this board due to internal SRAM constraints

docs/porting/capability-flags.md

@@ -24,6 +24,10 @@ Keep the two in sync. The pattern in `caps.cpp` does this for you:
 | `BOARD_HAS_BATTERY`            | 0       | Whether PMU battery measurement is meaningful on this board. UI hides the battery indicator when false. |
 | `BOARD_HAS_IO_EXPANDER`        | 0       | Whether an IO expander gates display / touch reset lines. Doesn't directly gate any code path — but signals to the porter that `board_init()` must release the expander before `display_hal_init()`. |
 
+## Build-flag macros
+
+`BOARD_HAS_PSRAM` is set as a `-D` build flag in `platformio.ini` (not in `board.h`) on chips with external PSRAM wired up. Shared code (`main.cpp`, `splash.cpp`) and per-board display drivers use it to choose between `MALLOC_CAP_SPIRAM` (large buffers) and `MALLOC_CAP_INTERNAL` (small buffers, partial-render LVGL, splash canvas capped at ~80 KB, screenshot capture disabled). New ESP32-C6 / ESP32-C3 ports must leave this undefined.
+
 ## Future capabilities
 
 Add a new flag when: