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/loosefolder.py

@@ -54,7 +54,7 @@ def _iter_local(path: Path, pattern: str):
       * Reject directories (a folder named `audio.wem` or `lead.xml`
         would otherwise be matched by glob and break downstream
         readers / converters).
-      * Reject symlinks escaping the folder so a crafted CDLC can't
+      * Reject symlinks escaping the folder so a crafted custom song can't
         smuggle external content into the scan.
     """
     root = path.resolve()
@@ -176,7 +176,7 @@ def _arr_type_from_filename(stem: str) -> tuple:
 
 
 def _parse_xml_meta(xml_path: Path) -> dict:
-    """Parse a Rocksmith arrangement XML and return song-level metadata."""
+    """Parse a the source game arrangement XML and return song-level metadata."""
     try:
         root = ET.parse(str(xml_path)).getroot()
         if root.tag != "song":