feat(tools): a weapon hold lab for tuning each character's grip in 3D

I have been guessing at numbers that want an eye on them. This is the
environment to set them instead.

  godot --path . res://debug/weapon_lab.tscn

Pick a character, pick a weapon, pick a pose, drag sliders, press Save.
Thirteen knobs — weapon size, support-hand distance along the barrel, grip
to buttstock, muzzle pitch at low ready, both wrist rolls, three finger
curl amounts, the stock pocket for low-ready and for aiming, and both elbow
poles. Three markers show the points being solved for: red trigger grip,
green support hand, blue buttstock. If a hand is not ON its marker the IK
could not reach it, which is a different problem from the marker being in
the wrong place, and the two used to be indistinguishable.

Results land in assets/characters/weapon_holds.json, resolved in layers so
a number can be set once and contradicted where it matters:

  defaults              every character, every weapon
  skins.<skin>._all     this character, every weapon
  skins.<skin>.<weapon> this character, this weapon

An empty file means "use what the code derives", so the game runs exactly
as before until something is actually tuned. The knobs that CAN be derived
from the skeleton still are — mount rotation, wrist frame, weapon size —
and their sliders read "auto" at zero rather than silently overriding.

Two things this had to get right to be honest rather than merely present.
Slider defaults come from the same table as the code defaults, because a
slider parked at 0 beside a code default of 1.0 means the first touch of
that slider silently switches finger curl off. And the markers are
depth-tested: drawn through the body they look like they are floating in
front of the chest when they are really behind an arm, which is the exact
wrong impression for judging whether a hand is on its target.

`-- shot <path> [skin] [weapon]` renders one framed close-up and quits, so
the lab can be checked without a human at the controls — which is how both
of those bugs were caught.

Co-Authored-By: Claude Opus 5 <[email protected]>

debug/weapon_lab.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3 uid="uid://cwpnlabweapon01"]
+
+[ext_resource type="Script" path="res://debug/weapon_lab.gd" id="1_lab"]
+
+[node name="WeaponLab" type="Node3D"]
+script = ExtResource("1_lab")