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>

variants/m5stack_cardputer_adv/platformio.ini

@@ -97,35 +97,6 @@ lib_deps =
   ${M5Stack_Cardputer_ADV.lib_deps}
   densaugeo/base64 @ ~1.4.0
 
-[env:M5Stack_Cardputer_ADV_companion_solo_dual]
-extends = M5Stack_Cardputer_ADV
-build_flags =
-  ${M5Stack_Cardputer_ADV.build_flags}
-  -I examples/companion_radio/ui-new
-  -D MAX_CONTACTS=350
-  -D MAX_GROUP_CHANNELS=40
-  -D BLE_PIN_CODE=123456
-  -D DUAL_SERIAL=1
-  -D OFFLINE_QUEUE_SIZE=256
-  -D FIRMWARE_SOLO_BUILD=1
-  -D MESHCORE_VERSION='"1.17"'
-  -D UI_SENSORS_PAGE=1
-  ; misc-fixed 6x9 font (full Latin/Greek/Cyrillic), same as the Wio L1/Heltec
-  ; solo builds. Solo only: it costs ~14 KB of flash, and this board's own
-  ; QWERTY keyboard is exactly the case that benefits from typing those
-  ; alphabets and having them render as themselves instead of blocks.
-  -D OLED_MISC_FIXED_FONT=1
-;  -D MESH_PACKET_LOGGING=1
-;  -D MESH_DEBUG=1
-build_src_filter = ${M5Stack_Cardputer_ADV.build_src_filter}
-  +<helpers/esp32/*.cpp>
-  +<helpers/ui/MomentaryButton.cpp>
-  +<../examples/companion_radio/*.cpp>
-  +<../examples/companion_radio/ui-new/*.cpp>
-lib_deps =
-  ${M5Stack_Cardputer_ADV.lib_deps}
-  densaugeo/base64 @ ~1.4.0
-
 [env:M5Stack_Cardputer_ADV_repeater]
 extends = M5Stack_Cardputer_ADV
 build_flags =