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.

lib/progression.py

@@ -331,7 +331,7 @@ def load_content(root) -> tuple[dict, list]:
 def instrument_for_arrangement(arr_entry) -> str:
     """Map a library arrangement entry to a progression instrument.
 
-    PSARC/loose entries carry ``type`` (lead/rhythm/bass/combo); sloppaks may
+    archive/loose entries carry ``type`` (lead/rhythm/bass/combo); sloppaks may
     only carry ``name``. Vocals are recognised so they never count toward
     guitar challenges; everything else defaults to guitar.
     """
@@ -345,11 +345,11 @@ def instrument_for_arrangement(arr_entry) -> str:
         return "drums"
     if arr_type in ("piano", "keys"):
         return "keys"
-    # Check name before committing to a guitar type — legacy PSARC keys
+    # Check name before committing to a guitar type — legacy archive keys
     # arrangements often carry a generic type (lead/rhythm/combo) but have a
     # name like "Keys" or "Piano".  Name overrides the generic type for all
     # well-known non-guitar instruments so that scored keys runs advance the
-    # keys path and quests even when the Rocksmith XML type was not updated.
+    # keys path and quests even when the the source game XML type was not updated.
     if "bass" in name:
         return "bass"
     if "drum" in name or "percussion" in name: