Avatar: tune Roblox proportions + walk cycle from real rendered screenshots

Now that cloudbison can render (Radeon GPU via Xvfb, or llvmpipe software),
added scripts/tools/screenshot_rig.gd + scenes/dev/ so a scene can be booted,
run a few frames, and its rendered frame saved to PNG -- caught three things
the headless smoke never could:

- limbs floated detached from the torso: widened the torso and overlapped the
  arm/leg boxes into it, and pulled limb depth in behind the torso front so
  they stop z-fighting.
- proportions were scrawny: reworked to Roblox R6 ratios (2-stud torso, 2-stud
  arms/legs, 1-stud head) with the head clearly pegged on top.
- the walk swing was a 40-deg lunge: STRIDE 0.7->0.42 rad, separate 0.32 arm
  swing, KNEE 1.0->0.5. Reads as a walk now, not a crouch.

test_level lighting was near-black on the shaded side (ambient was disabled) --
colour ambient at 0.8 + a stronger key light, so the avatar reads in-world.

avatar_preview.tscn (AGP_PREVIEW_WALK=1) renders the assembled avatar walking
in place for quick visual checks.

Verified: avatar smoke 14/14 source + exported pack; fresh-clone boot PASS;
eyeballed in rendered screenshots (game scene + walk sequence).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scenes/worlds/test_level.tscn

@@ -46,10 +46,10 @@ subdivisions_height = 8
 
 [sub_resource type="Environment" id="WorldEnvironment_env"]
 background_mode = 1
-background_color = Color(0.4, 0.6, 0.8, 1)
+background_color = Color(0.45, 0.62, 0.82, 1)
 ambient_light_source = 2
-ambient_light_color = Color(0.55, 0.6, 0.68, 1)
-ambient_light_energy = 0.5
+ambient_light_color = Color(0.7, 0.75, 0.82, 1)
+ambient_light_energy = 0.8
 
 [node name="TestLevel" type="Node3D"]
 
@@ -110,7 +110,8 @@ shape = SubResource("SphereShape3D_prop5")
 mesh = SubResource("SphereMesh_prop5")
 
 [node name="Sun" type="DirectionalLight3D" parent="."]
-rotation = Vector3(-0.785398, 0, 0)
+rotation = Vector3(-0.95, -0.5, 0)
+light_energy = 1.3
 shadow_enabled = true
 
 [node name="WorldEnvironment" type="WorldEnvironment" parent="."]