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.
