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/note_detection_domain.test.js

@@ -172,7 +172,7 @@ test('diagnostics contribution is redaction-safe', async () => {
     const contribution = window.__diagnosticsContributions.get('note-detection-capability');
     assert.equal(contribution.schema, 'slopsmith.note_detection_capability.v1');
     const serialized = JSON.stringify(contribution);
-    assert.ok(!/Yamaha|deviceLabel|filename|\.sloppak|\.psarc/i.test(serialized), serialized);
+    assert.ok(!/Yamaha|deviceLabel|filename|\.sloppak|\.archive/i.test(serialized), serialized);
 });
 
 test('legacy setNoteStateProvider surface is wrapped and accounted', () => {
@@ -227,7 +227,7 @@ test('_contextSummary whitelists arrangement kind — unknown values are dropped
     // Arbitrary string must not appear in context summary.
     const open2 = await api.dispatch({
         capability: 'note-detection', command: 'open-binding',
-        source: 'caller', payload: { context: { arrangement: '/Users/victim/song.psarc' } },
+        source: 'caller', payload: { context: { arrangement: '/Users/victim/song.archive' } },
     });
     assert.equal(open2.outcome, 'handled');
     const snap2 = window.slopsmith.noteDetection.snapshot();