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/constants.gd

@@ -96,6 +96,9 @@ const FACING_INDICATOR_OFFSET: float = 14.0
 # two columns of text comfortably, leaves world game visible alongside.
 const DIALOGUE_MAX_WIDTH: int = 640
 
+# Default camera zoom — used as fallback when get_camera_2d() returns null
+const CAMERA_DEFAULT_ZOOM: Vector2 = Vector2(2.0, 2.0)
+
 # #517: Implant UI font color grading — avoid pure white, project through a lens
 const IMPLANT_TEXT_COLOR: Color = Color("#E0F7FA")   # Cyan-white — primary text
 const IMPLANT_TEXT_DIM: Color = Color("#9EBFC4")     # Dimmed variant — secondary text