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>

.github/workflows/build-solo-firmwares.yml

@@ -26,7 +26,7 @@ jobs:
       - name: List solo_dual envs
         id: list-envs
         run: |
-          envs=$(grep -rh '^\[env:' platformio.ini variants/ \
+          envs=$(grep -rh '^\[env:' solo/ \
             | sed 's/\[env://;s/\]//' \
             | grep '_solo_dual$' \
             | jq -R -s -c 'split("\n") | map(select(length > 0))')

.github/workflows/pr-build-check.yml

@@ -7,6 +7,7 @@ on:
       - 'src/**'
       - 'examples/**'
       - 'variants/**'
+      - 'solo/**'
       - 'platformio.ini'
       - '.github/workflows/pr-build-check.yml'
   push:
@@ -15,6 +16,7 @@ on:
       - 'src/**'
       - 'examples/**'
       - 'variants/**'
+      - 'solo/**'
       - 'platformio.ini'
       - '.github/workflows/pr-build-check.yml'