Add finger IK system for the hand rig

FingerIK: per-finger hinge-constrained CCD solver over a local +Y bone
chain. Procedural curl seed (rest-relative) blended with optional IK
reach toward a target via ikWeight, plus a lateral DOF on the base joint
so targets can pull fingers slightly sideways and upward (back-of-hand).
Runs in edit mode via [ExecuteAlways] + an EditorApplication.update tick.

HandIK: auto-discovers the five fingers under the hand root, wires a
FingerIK per finger (thumb gets its own limits), exposes inspector target
slots, and drives all solves once per frame/tick. Pushes the rig curl
hinge axis (-X) and limits centrally so re-discovery corrects already
serialized components. Live ikWeight broadcast; non-destructive target
resolution (a FingerIK's own target survives an empty manager slot).
Input signature is additive so every finger refreshes live while dragging.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/FingerIK.cs.meta

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