fix: foot-IK pelvis drop ignores feet over drop-offs (+ solver perf)

[blugart-dev]

Summary

Fixes a foot-IK pelvis-drop bug where one foot over a gap/drop-off dragged the whole body down, plus removes the solver's per-frame allocations. Part of the 0.3.x finalization gate.

The bug (correctness)

The pelvis lowers to accommodate the lowest planted foot. But offset_* (which feed the pelvis drop) come from a ground raycast that can hit up to foot_ik_max_adjustment (0.5 m) below the foot. So a foot stepping out over a ledge/pit — its raycast hitting ground far below — was treated as a deep plant and pulled the pelvis down to foot_ik_max_pelvis_drop (0.35 m). That sank the body and lifted the other, genuinely-planted foot off its ground.

The fix

A foot now only informs the pelvis drop when it found ground the body can actually reach (within foot_ik_max_pelvis_drop). Feet over a gap (no raycast hit) or a deeper drop-off are excluded from the minf. Key property: offset_* feed only the pelvis drop — the per-leg IK solves already target the raw ground hit (gpos.y) — so legs still reach toward real ground; only the spurious whole-body sink is removed. On flat/normal terrain both feet are supported, so behavior is identical (existing flat-ground tests unchanged).

Regression test

test_pelvis_ignores_foot_over_dropoff builds split ground — solid under the left foot, a ~1 m drop-off under the right — and asserts the left foot still plants and the pelvis stays near neutral (> -0.1). On the old code the pelvis converged to -0.35, so the test genuinely distinguishes the fix.

Solver per-frame perf (no behavior change)

• The target-override Dictionary handed to the SpringResolver each solve is now a reused buffer instead of a fresh {}. Safe because the controller's solve and the spring's read never interleave within a physics frame (each node's _physics_process runs to completion in turn).
• Animation bone globals are memoized per solve (_anim_global / _anim_cache), so the hip/leg reads and the full-body shift no longer re-walk shared parent chains.

Validation

• Headless import: clean
• GUT: 113/113, 3× stable, no orphans (was 112; +1 new test)
• Scene-smoke (--quit-after 150): clean on foot_ik_demo

Visual sign-off

Wants an in-editor pass on varied terrain before merge — specifically: stand a character with one foot on a platform edge and the other over a gap/lower step, and confirm the body no longer sinks/crouches when a foot overhangs a drop-off, while normal uneven ground (stairs/slopes within reach) still drops the pelvis as before.

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