refactor(firmware): per-panel folder layout + parametrized config.h

Reorganizes the tree so adding a new panel is purely additive — drop in a
new src/panels/{vendor}/v{N}/ folder and a new platformio.ini env block,
no surgery to existing files.

Layout:
  src/                              shared across all panels
  src/panels/waveshare73/v1/        V1 driver, version, README
  data/waveshare73-v1/              LittleFS payload at this panel's size

src/config.h still defines the panel-agnostic bits (NVS keys, color
palette, network, sync-fail border) but EPD_WIDTH / EPD_HEIGHT / pin
assignments now come from each env's -D flags. Strict #error guards in
production builds; native tests get the V1 defaults via UNIT_TEST.

build_src_filter per env picks the right driver:
  waveshare73-v1   main + panels/waveshare73/v1/
  test-display     test_display + panels/waveshare73/v1/
  sim-yellow       sim_border + panels/waveshare73/v1/
  sim-red          sim_border + panels/waveshare73/v1/
  native-test      unchanged

When V2 hardware lands, the diff is a new env block, a new
src/panels/waveshare133/v1/epd_driver.cpp, and regenerated screens at
data/waveshare133-v1/. Existing V1 envs stay frozen — re-flashing old
units remains a one-liner.

scripts/gen_screens.py takes --panel to target the correct
data/{panel}/ subfolder; defaults to waveshare73-v1.

29/29 native tests pass. All four hardware envs build clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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