t2m: foot-contact cleanup — detect ground contact + IK-pin feet post-pass

[fernandotonon]

Part of epic #837. Retargeted clips slide/float feet on rigs whose proportions differ from the source (cartoon characters especially) — the direction retarget (PR #843) transfers bone directions, not world foot positions.

Plan

• Contact detection on the generated clip: per foot role, frames where the foot's world height is within a band of the clip's ground level AND its horizontal velocity is near zero → contact spans.
• Pinning: during each contact span, lock the foot's world position to its span-start position with a 2-bone analytic IK (hip–knee–foot; knee axis from the existing pose), blending in/out over a few frames at span edges.
• Post-process on the animation tracks (same architecture as the Retarget post-processing controls: Mixamo-style arm-space slider #854 arm-space slider: idempotent, undoable, qtme.* binding to store state).
• Surfaces: on-by-default toggle in generation (GUI checkbox, CLI --no-foot-pin, MCP arg); standalone application to an existing clip.
• Pure-data core (contact detection + 2-bone IK) unit-tested without GL.

Acceptance

• Fox walk: feet visibly plant instead of skating; no knee pops (blend windows).
• Idempotent re-apply; legacy/template/model paths all supported.