refactor(solo): move solo build configs into their own solo/ folder; one build per board; document all build flags

Solo's `_solo_dual` env for each board lived inside variants/<board>/, mixed
in with the shared upstream board configs (repeater, room server, plain
companion) that every MeshCore fork carries. Split them out into their own
top-level solo/<board>/ folder -- each still `extends` the board base defined
in its original variants/<board>/platformio.ini, just no longer interleaved
with it file-wise.

- platformio.ini: extra_configs picks up solo/*/platformio.ini alongside
  variants/*/platformio.ini.
- build-solo-firmwares.yml: env-discovery grep now scans solo/ instead of
  variants/ (the release matrix is unchanged -- same 8 envs, same names).
- pr-build-check.yml: solo/** added to the trigger paths.
- GAT562 30S Mesh Kit: dropped the separate solo_ble env -- solo_dual is a
  strict superset (BLE still works, plus USB), and every other board only
  ever had one solo build to begin with.
- GAT562 Mesh Watch13: renamed solo_ble -> solo_dual (added DUAL_SERIAL=1) to
  match. No comment anywhere recorded whether this board's USB data lines
  are actually broken out to a connector -- DUAL_SERIAL compiles and works
  over BLE regardless, so worst case the USB half goes unused.
- Removed a dead AUTO_SHUTDOWN_MILLIVOLTS from the three solo envs that set
  it (Heltec V3/V4, T-Echo Lite+KeyShield): that macro is only ever read in
  the old ui-tiny UITask, never ui-new, which every solo build (these
  included) uses -- it did nothing on any of them. Left alone everywhere
  else it's set (non-solo companion envs elsewhere use ui-tiny/ui-orig, or
  the flag is legitimately read); out of scope here.
- Added docs/solo_features/build_flags.md: every optional -D flag a solo
  build understands (GPIO, CardKB/joystick, Hall-sensor cover lock, buzzer/
  vibration, GPS switch, display/battery tuning), verified against the code
  rather than the existing per-board comments, with what's already baked
  into every solo build kept separate from what's opt-in.
- README: doc index + Building from source section link to the new page;
  path references to the moved solo files updated to solo/<board>/.

All 8 solo_dual envs rebuilt clean after the move; native test suite (40
cases) unaffected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

solo/gat562_30s_mesh_kit/platformio.ini

@@ -0,0 +1,52 @@
+; Solo build for GAT562 30S Mesh Kit -- extends the board base defined in
+; variants/gat562_30s_mesh_kit/platformio.ini. Solo configs live in their own
+; top-level solo/ folder, separate from the shared upstream board variants in
+; variants/, so this repo's Solo-specific additions don't get mixed in with
+; configs other MeshCore forks/upstream also carry.
+;
+; Dual (BLE+USB) only -- the previous BLE-only solo_ble env was dropped since
+; dual is a strict superset (BLE still works, plus USB) and Solo standardises
+; on one build per board.
+[env:GAT562_30S_Mesh_Kit_solo_dual]
+extends = GAT562_30S_Mesh_Kit
+board_build.ldscript = boards/nrf52840_s140_v6_extrafs.ld
+board_upload.maximum_size = 712704
+; Optimise for size, not speed: the nRF52 Arduino core defaults to -Ofast
+; (≈ -O3 + -ffast-math), which bloats flash by ~200 KB on this feature-rich
+; solo build. -Os keeps every feature, fits comfortably, and is actually safer
+; for float (no fast-math reassociation). Speed is irrelevant for the UI/mesh.
+build_unflags = -Ofast
+build_flags =
+  ${GAT562_30S_Mesh_Kit.build_flags}
+  -I examples/companion_radio/ui-new
+  -D DISPLAY_CLASS=SSD1306Display
+  -D MAX_CONTACTS=350
+  -D MAX_GROUP_CHANNELS=40
+  -D BLE_PIN_CODE=123456
+  -D OFFLINE_QUEUE_SIZE=256
+  -D FIRMWARE_SOLO_BUILD=1
+  -D MESHCORE_VERSION='"1.17"'
+  -D UI_SENSORS_PAGE=1
+  -D UI_HAS_JOYSTICK_UPDOWN=1
+  -D ADVERT_NAME='"@@MAC"'
+  -D ENV_INCLUDE_GPS=1
+  ; misc-fixed 6x9 OLED font (full Latin/Greek/Cyrillic), same as the Wio L1
+  ; builds. Solo only: it costs ~14 KB of flash and the repeater/companion
+  ; builds have no keyboard to type those alphabets on.
+  -D OLED_MISC_FIXED_FONT=1
+  -D DUAL_SERIAL=1
+  -D ENABLE_SCREENSHOT
+  -Os
+  ; QSPIFLASH disabled: QSPI IO0/IO2 pins (P0.30/P0.28) conflict with joystick LEFT/UP
+  ; -D MESH_PACKET_LOGGING=1
+  ; -D MESH_DEBUG=1
+build_src_filter = ${GAT562_30S_Mesh_Kit.build_src_filter}
+  +<helpers/nrf52/SerialBLEInterface.cpp>
+  +<helpers/ui/buzzer.cpp>
+  +<../examples/companion_radio/*.cpp>
+  +<../examples/companion_radio/ui-new/*.cpp>
+lib_deps =
+  ${GAT562_30S_Mesh_Kit.lib_deps}
+  adafruit/RTClib @ ^2.1.3
+  densaugeo/base64 @ ~1.4.0
+  end2endzone/NonBlockingRTTTL@^1.3.0