fix(client): address PR #43 review — walk key mismatch + 5 suggestions

Fix SOUND_EVENT_ASSETS walk-speed keys to match actual filename
(sfx_footstep_metal_walk), add play_loop null guard, source indicator
colors from Constants, extract CAMERA_DEFAULT_ZOOM, document
consume-once semantics on close_sound_events.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/main.gd

@@ -201,6 +201,8 @@ func _process(_delta: float) -> void:
 # D-018 #125: Play close-range sound events — fired once per snapshot tick.
 # Each event is passed to AudioManager.play_sound_event() for 2D positional playback
 # on the WorldSFX bus. Events with no registered asset are silently skipped (D-038).
+# Consume-once: events are cleared after processing so they don't replay if
+# _process runs again before the next server tick (D-009 multiplayer-safe pattern).
 func _play_close_sound_events() -> void:
 	for evt in GameState.close_sound_events:
 		if not evt is Dictionary or not evt.has("x") or not evt.has("y"):