settings: add host instrument profiles (#753)

* settings: add host instrument profiles

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

* settings: add instrument pathway selection

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

* fix(settings): profile-aware saves/resets/switch, provider tunings, bass-5

Five regressions from the instrument-profiles rework:

1. save_settings canonicalized profiles on EVERY save -> empty/unrelated POST
   froze default profiles into config.json (broke
   test_empty_post_preserves_all_existing_keys). Gate on the save touching
   instrument settings; GET already virtualizes profiles.
2. pathway is profile-mirrored, so the Gameplay reset (flat-key delete) was a
   no-op. reset_settings now resets pathway inside the persisted profiles too.
3. Per-profile tuning validation rejected provider/custom tunings (tuner
   plugin, /api/tunings). _valid_tuning_for_key now accepts a name unknown to
   every built-in table while still rejecting a built-in misapplied to the
   wrong key.
4. First-migration overwrote an explicit active_instrument_profile with the
   legacy-inferred one, so a fresh-config switch to 'bass' was lost. Use
   setdefault so an explicit request wins.
5. Pre-existing test_instrument_fields_persist used bass-5 + 'Drop D' (a
   4-string tuning). Updated to the valid 'Drop A'.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(settings): partial-merge instrument_profiles; clamp tuning on string-count switch

Two partial-update follow-ups:
- save_settings normalized a POSTed instrument_profiles by FILLING every omitted
  profile with defaults and replacing wholesale, so a one-profile update reset
  the others. Validate each PROVIDED profile individually and merge the partial
  over the persisted set inside the lock — /api/settings is partial-merge.
- the string-count picker posted only string_count, so the backend silently
  reset a now-invalid tuning to Standard while the UI kept the old value
  (settings/tuner desync). Clamp + post the valid tuning too, mirroring the
  instrument-switch path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/v3/settings.js

@@ -28,7 +28,7 @@
     var RESET_MAP = {
         gameplay: {
             server: ['master_difficulty', 'av_offset_ms', 'miss_penalty',
-                'fail_behavior', 'countdown_before_song', 'default_arrangement'],
+                'fail_behavior', 'countdown_before_song', 'default_arrangement', 'pathway'],
             local: ['lefty', 'autoplayExit', 'showUpNext', 'confirmExitSong', 'arrangementNamingMode', 'countdownBeforeSong'],
             after: function () {
                 // Left-handed is held on the highway object, not re-derived