Purge external-format terminology from code, tests and docs

Reword comments/docstrings/strings and rename identifiers that referenced
the external game and its file formats:

- format-id "psarc" -> "archive"; local vars psarc_path -> song_path,
  psarc_base -> tone_base
- lyrics provenance value "sng" -> "notechart" (legacy "sng" still accepted)
- highway_3d fret-ghost scope value "rocksmith" -> "chords" (invalid/legacy
  values fall back to the default, preserving behaviour)
- neutralise references in prose, test names/data, .gitattributes and docs

No functional change beyond the renamed identifiers; all Python compiles.

tests/js/audio_effects_host.test.js

@@ -817,7 +817,7 @@ test('mapping helpers call core mapping API with provider-tagged payloads', asyn
 
     const saved = await window.slopsmith.audioEffects.upsertMapping({
         song_key: 'settings-v1-song',
-        filename: 'Artist - Song_p.psarc',
+        filename: 'Artist - Song_p.archive',
         tone_key: 'Dist',
         provider_id: 'rig-builder',
         provider_ref: 'chain:99',
@@ -880,7 +880,7 @@ test('bridge hits are safe and diagnosable', async () => {
             routeKey: 'desktop-main',
             bridgeId: 'audio-effects.legacy-nam-routing',
             pluginId: 'rig_builder',
-            legacySurface: 'fetch /Users/example/song.psarc token=abc123',
+            legacySurface: 'fetch /Users/example/song.archive token=abc123',
         },
     });
     const dbResult = await api.dispatch({