Add industrial metal soundtrack with seamless loop

- assets/audio/music/zombie_industrial_loop.ogg: 57.60 s Ogg Vorbis loop
  generated locally with YuE2-3B on the Tesla V100 (score written for this
  track; vocals removed with HTDemucs). The loop is spliced at the track's
  structural repeat with a 0.6 s crossfade: measured seam dip <= 1.6 dB.
- scripts/main.gd: "Music" AudioStreamPlayer at -11 dB, started at boot on
  desktop and on the start click on Web (browser autoplay policy). It forces
  stream.loop at runtime because the Web staging step re-imports the OGG with
  loop=false.
- tools/test_music.gd: soundtrack test (loop, length, volume, playback,
  still playing after player death).
- docs/MUSIC.md: source, loop construction, verification, licence.
- docs/WEB-DEPLOY.md: release ab6054111f5919a8 notes.

Initial commit of the project.

addons/goldsrc/plugin.gd

@@ -0,0 +1,29 @@
+@tool
+extends EditorPlugin
+
+var bsp_importer: EditorImportPlugin
+var mdl_importer: EditorImportPlugin
+var spr_importer: EditorImportPlugin
+var wad_importer: EditorImportPlugin
+
+
+func _enter_tree() -> void:
+	bsp_importer = preload("res://addons/goldsrc/importer/bsp_importer.gd").new()
+	mdl_importer = preload("res://addons/goldsrc/importer/mdl_importer.gd").new()
+	spr_importer = preload("res://addons/goldsrc/importer/spr_importer.gd").new()
+	wad_importer = preload("res://addons/goldsrc/importer/wad_importer.gd").new()
+	add_import_plugin(bsp_importer)
+	add_import_plugin(mdl_importer)
+	add_import_plugin(spr_importer)
+	add_import_plugin(wad_importer)
+
+
+func _exit_tree() -> void:
+	remove_import_plugin(bsp_importer)
+	remove_import_plugin(mdl_importer)
+	remove_import_plugin(spr_importer)
+	remove_import_plugin(wad_importer)
+	bsp_importer = null
+	mdl_importer = null
+	spr_importer = null
+	wad_importer = null