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/autoloads/audio_manager.gd

@@ -142,6 +142,8 @@ func play_loop(asset_key: String, bus: String = BUS_AMBIENT) -> AudioStreamPlaye
 		return null
 	stop_loop(asset_key)
 	var loop_stream := stream.duplicate() as AudioStream
+	if loop_stream == null:
+		return null
 	_enable_loop(loop_stream)
 	var player := AudioStreamPlayer.new()
 	player.stream = loop_stream
@@ -175,10 +177,10 @@ func stop_all_loops() -> void:
 # No asset for Voice events in v0.1 — play_sound_event no-ops gracefully
 # (D-038: "renders as audio if asset exists, or visual indicator + monologue if not").
 const SOUND_EVENT_ASSETS: Dictionary = {
-	"Footstep":       "sfx_footstep_metal",
-	"FootstepWalk":   "sfx_footstep_metal",
-	"FootstepCareful":"sfx_footstep_metal",
-	"FootstepCrouch": "sfx_footstep_metal",
+	"Footstep":       "sfx_footstep_metal_walk",
+	"FootstepWalk":   "sfx_footstep_metal_walk",
+	"FootstepCareful":"sfx_footstep_metal_walk",  # D-053: same asset until stance-differentiated audio lands
+	"FootstepCrouch": "sfx_footstep_metal_walk",  # D-053: same asset until stance-differentiated audio lands
 	"FootstepSprint": "sfx_footstep_metal_run",
 	"FootstepRun":    "sfx_footstep_metal_run",
 }