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/browser/default-arrangement-pin.spec.ts

@@ -70,7 +70,7 @@ test('player arrangement pin saves the selected arrangement name', async ({ page
     select.value = '2';
     // @ts-ignore - browser app namespace
     window.slopsmith.currentSong = {
-      filename: 'demo.psarc',
+      filename: 'demo.archive',
       arrangement: 'Rhythm',
       arrangementIndex: 2,
       arrangements,
@@ -138,7 +138,7 @@ test('player arrangement pin preserves non-built-in arrangement names', async ({
     select.value = '2';
     // @ts-ignore - browser app namespace
     window.slopsmith.currentSong = {
-      filename: 'demo.psarc',
+      filename: 'demo.archive',
       arrangement: 'Rhythm',
       arrangementIndex: 2,
       arrangements,
@@ -192,7 +192,7 @@ test('failed settings save does not mark arrangement default as persisted', asyn
     select.value = '2';
     // @ts-ignore - browser app namespace
     window.slopsmith.currentSong = {
-      filename: 'demo.psarc',
+      filename: 'demo.archive',
       arrangement: 'Rhythm',
       arrangementIndex: 2,
       arrangements,