fix(assets): PR #57 review — docs, uid, citation fixes

- Fix client sprite naming in README (files are _64.png, not .png)
- Clarify --path working directory (renderer/ from repo root)
- Remove /sprite-gen reference (skill not in branch yet)
- Add D-043 citation on outline color in render_export.gd
- Add uid to wall_bar_green.tscn for reproducible imports
- Note wall_bar_green has no rendered sprites yet
- Note npc_generic capsule symmetry is by design (D-044)
- Add outline + D-033 tint compatibility guidance

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

renderer/models/wall_bar_green.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=3 format=3]
+[gd_scene load_steps=3 format=3 uid="uid://wallbargreen001"]
 
 [ext_resource type="Texture2D" path="res://textures/wall_bar_green_panels.png" id="1_wall_texture"]