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/heltec_v3/platformio.ini

@@ -180,91 +180,6 @@ lib_deps =
   ${Heltec_lora32_v3.lib_deps}
   densaugeo/base64 @ ~1.4.0
 
-; Solo build: the full standalone on-device UI (message history, contacts, map,
-; tools) rather than a screen that only mirrors the phone app. Dual transport --
-; the companion app can attach over BLE or USB serial, whichever it finds, and
-; BLE wins while both are live.
-;
-; This firmware needs a real input device: the solo UI cannot be driven from the
-; PRG button alone. Both supported ones are wired up in target.cpp and enabled
-; below, so either is enough on its own and having both is fine. The pins are
-; defaults picked from what this board leaves free -- change them to match how
-; the device is actually built. Pins with nothing attached read as not-pressed,
-; so the unused half costs nothing.
-;
-;  * CardKB (M5Stack I2C keyboard, addr 0x5F) on a second I2C bus. Any two free
-;    GPIOs -- NOT 17/18, the OLED already owns those on Wire. It's probed at
-;    boot, so a build with these set still runs fine with nothing plugged in.
-;    Pair it with Settings > Keyboard > Ext. KB = Compact, which is designed to
-;    need no joystick at all. The same bus is scanned for environment sensors.
-;
-;  * Wired joystick: four direction pins plus a fifth "press" contact. The
-;    press contact drives Enter (your thumb's already on the stick) via
-;    PIN_USER_BTN -- overridden below (a later -D wins, no -U needed: this whole
-;    project builds with -w) from the PRG-button default
-;    the base env sets, since PRG's GPIO0 can't be moved to the stick. PRG
-;    becomes Back instead (PIN_BACK_BTN), so it no longer needs its own
-;    separately-wired button. Each contact just shorts its pin to GND;
-;    target.cpp enables the internal pull-ups. Drop
-;    UI_HAS_JOYSTICK_UPDOWN for a left/right-only stick. JOYSTICK_ROTATION (0-3)
-;    rotates the mapping if the stick is mounted sideways -- it's also a runtime
-;    setting, so leave it out unless you want a different default.
-;    Comment the whole block out for a CardKB-only build. UI_HAS_JOYSTICK
-;    replaces the single-button handling rather than adding to it (UITask.cpp
-;    dispatches on '#if UI_HAS_JOYSTICK ... #elif defined(PIN_USER_BTN)'), which
-;    costs nothing here -- that fallback only feeds KEY_NEXT/KEY_PREV, which the
-;    solo screens don't act on.
-;
-; V3 and V4 are pin-compatible per Heltec's own docs, so the defaults below are
-; carried over from the heltec_v4_companion_solo_dual env, where this exact
-; CardKB + joystick combination is confirmed working on real hardware -- still
-; worth checking against your own V3 module before soldering, but not a blind
-; guess like the earlier SDA 41/SCL 42 default was. PIN_GPS_RX=47/TX=48/EN=26
-; above are dead defines on V3 (ENV_INCLUDE_GPS is never set to 1 in this file),
-; so they don't conflict with reusing 47/48 here despite the name.
-[env:Heltec_v3_companion_solo_dual]
-extends = Heltec_lora32_v3
-build_flags =
-  ${Heltec_lora32_v3.build_flags}
-  -I examples/companion_radio/ui-new
-  -D MAX_CONTACTS=350
-  -D MAX_GROUP_CHANNELS=40
-  -D DISPLAY_CLASS=SSD1306Display
-  -D BLE_PIN_CODE=123456   ; dynamic, random PIN
-  -D DUAL_SERIAL=1         ; companion app over BLE *or* USB serial
-  -D AUTO_SHUTDOWN_MILLIVOLTS=3400
-  -D OFFLINE_QUEUE_SIZE=256
-  -D FIRMWARE_SOLO_BUILD=1
-  -D UI_SENSORS_PAGE=1
-  -D OLED_MISC_FIXED_FONT=1   ; full Latin/Greek/Cyrillic 6x9 font, ~14 KB flash
-  -D ENABLE_SCREENSHOT        ; same as the other solo builds — Solo Tools reads the framebuffer over USB
-  ; CardKB on the second I2C bus. Harmless with nothing plugged in.
-  -D ENV_PIN_SDA=3
-  -D ENV_PIN_SCL=4
-  ; Wired joystick — comment the block out for a CardKB-only build. The
-  ; press contact (Enter) overrides PIN_USER_BTN from its PRG default;
-  ; PRG becomes Back (PIN_BACK_BTN) instead.
-  -D UI_HAS_JOYSTICK=1
-  -D UI_HAS_JOYSTICK_UPDOWN=1
-  -D JOYSTICK_UP=23
-  -D JOYSTICK_DOWN=6
-  -D JOYSTICK_LEFT=47
-  -D JOYSTICK_RIGHT=48
-  -D PIN_BACK_BTN=0     ; PRG button
-  -D PIN_USER_BTN=33    ; joystick press contact -- Enter, overrides the PRG default above
-;  -D PIN_BUZZER=<gpio>
-build_src_filter = ${Heltec_lora32_v3.build_src_filter}
-  +<helpers/ui/SSD1306Display.cpp>
-  +<helpers/ui/MomentaryButton.cpp>
-  +<helpers/ui/buzzer.cpp>
-  +<helpers/esp32/*.cpp>
-  +<../examples/companion_radio/*.cpp>
-  +<../examples/companion_radio/ui-new/*.cpp>
-lib_deps =
-  ${Heltec_lora32_v3.lib_deps}
-  densaugeo/base64 @ ~1.4.0
-  end2endzone/NonBlockingRTTTL@^1.3.0
-
 [env:Heltec_v3_companion_radio_wifi]
 extends = Heltec_lora32_v3
 build_flags =