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/test_playlists_api.py

@@ -46,28 +46,28 @@ def test_create_requires_name(client):
 
 
 def test_add_remove_reorder_persists(client, server):
-    for fn in ("a.psarc", "b.psarc", "c.psarc"):
+    for fn in ("a.archive", "b.archive", "c.archive"):
         server.meta_db.put(fn, 0, 0, {})
     pid = client.post("/api/playlists", json={"name": "Set"}).json()["id"]
-    client.post(f"/api/playlists/{pid}/songs", json={"filename": "a.psarc"})
-    client.post(f"/api/playlists/{pid}/songs", json={"filename": "b.psarc"})
-    client.post(f"/api/playlists/{pid}/songs", json={"filename": "c.psarc"})
+    client.post(f"/api/playlists/{pid}/songs", json={"filename": "a.archive"})
+    client.post(f"/api/playlists/{pid}/songs", json={"filename": "b.archive"})
+    client.post(f"/api/playlists/{pid}/songs", json={"filename": "c.archive"})
     songs = client.get(f"/api/playlists/{pid}").json()["songs"]
-    assert [s["filename"] for s in songs] == ["a.psarc", "b.psarc", "c.psarc"]
+    assert [s["filename"] for s in songs] == ["a.archive", "b.archive", "c.archive"]
     # reorder
-    client.post(f"/api/playlists/{pid}/reorder", json={"order": ["c.psarc", "a.psarc", "b.psarc"]})
+    client.post(f"/api/playlists/{pid}/reorder", json={"order": ["c.archive", "a.archive", "b.archive"]})
     songs2 = client.get(f"/api/playlists/{pid}").json()["songs"]
-    assert [s["filename"] for s in songs2] == ["c.psarc", "a.psarc", "b.psarc"]
+    assert [s["filename"] for s in songs2] == ["c.archive", "a.archive", "b.archive"]
     # remove
-    client.request("DELETE", f"/api/playlists/{pid}/songs/b.psarc")
+    client.request("DELETE", f"/api/playlists/{pid}/songs/b.archive")
     songs3 = client.get(f"/api/playlists/{pid}").json()["songs"]
-    assert [s["filename"] for s in songs3] == ["c.psarc", "a.psarc"]
+    assert [s["filename"] for s in songs3] == ["c.archive", "a.archive"]
 
 
 def test_saved_for_later_toggle_and_protection(client):
     # Toggle creates the system playlist on first use.
-    assert client.post("/api/saved/toggle", json={"filename": "x.psarc"}).json() == {"saved": True}
-    assert client.post("/api/saved/toggle", json={"filename": "x.psarc"}).json() == {"saved": False}
+    assert client.post("/api/saved/toggle", json={"filename": "x.archive"}).json() == {"saved": True}
+    assert client.post("/api/saved/toggle", json={"filename": "x.archive"}).json() == {"saved": False}
     saved = next(p for p in client.get("/api/playlists").json() if p["system_key"] == "saved_for_later")
     # Cannot delete or rename the system playlist.
     assert client.delete(f"/api/playlists/{saved['id']}").status_code == 400
@@ -76,12 +76,12 @@ def test_saved_for_later_toggle_and_protection(client):
 
 def test_continue_session(client, server):
     assert client.get("/api/session/continue").json() is None
-    for fn in ("one.psarc", "two.psarc"):
+    for fn in ("one.archive", "two.archive"):
         server.meta_db.put(fn, 0, 0, {})
-    client.post("/api/stats", json={"filename": "one.psarc", "score": 100, "accuracy": 0.5, "lastPlayPosition": 12.0})
-    client.post("/api/stats", json={"filename": "two.psarc", "score": 200, "accuracy": 0.7, "lastPlayPosition": 30.0})
+    client.post("/api/stats", json={"filename": "one.archive", "score": 100, "accuracy": 0.5, "lastPlayPosition": 12.0})
+    client.post("/api/stats", json={"filename": "two.archive", "score": 200, "accuracy": 0.7, "lastPlayPosition": 30.0})
     cont = client.get("/api/session/continue").json()
-    assert cont["filename"] == "two.psarc"
+    assert cont["filename"] == "two.archive"
     assert cont["last_position"] == 30.0
     assert "art_url" in cont and "title" in cont
 
@@ -89,8 +89,8 @@ def test_continue_session(client, server):
 def test_add_song_to_missing_playlist_is_404(client, server):
     # add_playlist_song() must not insert an orphan row for a non-existent
     # playlist (the concurrent-delete TOCTOU); it returns None → handler 404s.
-    assert server.meta_db.add_playlist_song(999999, "x.psarc") is None
-    r = client.post("/api/playlists/999999/songs", json={"filename": "x.psarc"})
+    assert server.meta_db.add_playlist_song(999999, "x.archive") is None
+    r = client.post("/api/playlists/999999/songs", json={"filename": "x.archive"})
     assert r.status_code == 404
 
 
@@ -98,11 +98,11 @@ def test_playlist_hides_dead_songs_when_library_populated(client, server):
     # A playlist song whose file no longer exists is hidden from contents + count
     # (mirrors the stats read-filter), but only while the library is populated.
     db = server.meta_db
-    db.put("live.psarc", 0, 0, {"title": "Live"})
+    db.put("live.archive", 0, 0, {"title": "Live"})
     pid = client.post("/api/playlists", json={"name": "P"}).json()["id"]
-    client.post(f"/api/playlists/{pid}/songs", json={"filename": "live.psarc"})
-    client.post(f"/api/playlists/{pid}/songs", json={"filename": "ghost.psarc"})  # never in songs
+    client.post(f"/api/playlists/{pid}/songs", json={"filename": "live.archive"})
+    client.post(f"/api/playlists/{pid}/songs", json={"filename": "ghost.archive"})  # never in songs
     pl = client.get(f"/api/playlists/{pid}").json()
     names = [s["filename"] for s in pl["songs"]]
-    assert "live.psarc" in names and "ghost.psarc" not in names
+    assert "live.archive" in names and "ghost.archive" not in names
     assert [p for p in client.get("/api/playlists").json() if p["id"] == pid][0]["count"] == 1