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

scripts/avatar/limb_animator.gd

@@ -10,8 +10,9 @@ extends Node
 ## avatar's Skeleton3D. Bones are addressed by canonical name, so this works
 ## for the local player and for snapshot-driven remote players alike.
 
-const STRIDE := 0.7          ## peak hip/shoulder swing, radians
-const KNEE := 1.0            ## peak knee bend, radians
+const STRIDE := 0.42         ## peak hip swing, radians (~24 deg)
+const ARM_SWING := 0.32      ## peak shoulder swing, radians
+const KNEE := 0.5            ## peak knee bend, radians
 const CYCLE := 1.9           ## phase (radians) advanced per metre travelled
 const BLEND := 12.0          ## ease rate toward the target pose
 const MOVE_EPS := 0.35       ## m/s below which we treat the avatar as idle
@@ -52,11 +53,12 @@ func _process(delta: float) -> void:
 
 	var s := sin(_phase)
 	var swing := s * STRIDE * _amount
+	var arm := s * ARM_SWING * _amount
 
 	_pitch("LeftUpperLeg", swing)
 	_pitch("RightUpperLeg", -swing)
-	_pitch("LeftUpperArm", -swing)
-	_pitch("RightUpperArm", swing)
+	_pitch("LeftUpperArm", -arm)
+	_pitch("RightUpperArm", arm)
 	# Knee folds only while that leg is swinging back (negative pitch phase).
 	_pitch("LeftLowerLeg", maxf(0.0, -s) * KNEE * _amount)
 	_pitch("RightLowerLeg", maxf(0.0, s) * KNEE * _amount)