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/dev/avatar_preview.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3 uid="uid://c0avpre7dev0rig"]
+
+[ext_resource type="Script" path="res://scripts/tools/avatar_preview.gd" id="1"]
+
+[node name="AvatarPreview" type="Node3D"]
+script = ExtResource("1")

scenes/dev/screenshot_rig.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3 uid="uid://bqs8xk2rig0dev"]
+
+[ext_resource type="Script" path="res://scripts/tools/screenshot_rig.gd" id="1"]
+
+[node name="ScreenshotRig" type="Node"]
+script = ExtResource("1")