tools: measure limb deformation instead of eyeballing it

Adds debug/limb_deform_check.gd, which skins the leg vertices by hand and
compares them against the rest pose across a sweep of run, walk, jump, fall,
slide and dash. It reports lengthwise stretch (bending a knee can only shrink
this, so growth is genuine stretching) and cross-section loss (the linear-blend
"candy wrapper" collapse at a bent joint).

No gameplay or character code changes in this commit. Current readings on
Taila, through the full runtime stack:

  lengthwise stretch   1.05 worst (Body, ClothB, ClothCAndW), 1.03 outline shell
  cross-section        0.93 knee, 0.84 hip, 0.91 ankle

Sanity check on the method: the measured rest span of the leg vertex cloud is
0.818 m against 0.843 m of thigh+shin bone length, so the skinning maths is
producing real positions rather than plausible-looking noise.

That is ordinary linear-blend skinning. Combined with the earlier findings — no
bone pose basis deviating from a pure rotation by more than 0.00001, and no
bone length drifting by more than 0.0000 m — the leg geometry is not squashing
or stretching in any measurable way, so there is nothing here to fix blind.
Committing the instrument so the next round starts from a number.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

debug/limb_deform_check.gd.uid

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