[Feature] Add foot IK with analytic two-bone solver

[miogds]

Summary

Next link in the animation chain after inertialized transitions (#1125) and root motion (#1135): foot IK. Clips are authored against a flat ground plane; on real terrain feet float or sink. With foot IK enabled — opt-in per entity, default off, zero cost when disabled — the ground beneath each configured ankle is sampled every frame and the hip and knee are adjusted with a closed-form two-bone solver so the foot lands on it.

API

setFootIKChains(entityId: player, chains: [
    FootIKChainDescriptor(hipPath: "root/Hips/LeftUpperLeg", kneePath: ".../LeftLowerLeg", anklePath: ".../LeftFoot"),
    FootIKChainDescriptor(hipPath: "root/Hips/RightUpperLeg", kneePath: ".../RightLowerLeg", anklePath: ".../RightFoot"),
])
setFootIKEnabled(entityId: player, enabled: true)
setFootIKGroundQuery(entityId: player, query: myHeightfieldQuery)   // optional override; nil restores the default

• TwoBoneIK: Holden-style analytic solver — law-of-cosines bend plus aim rotation, expressed in each joint's local frame. Composition order is aim-then-bend in local post-multiplication (equivalent to bend-then-aim in world space); the wrong order is invisible whenever hint, chain, and target are coplanar, so the tests deliberately cover the non-coplanar case. Unreachable targets clamp to full extension; straight chains take the bend plane from a hint.
• FootIK: chains are resolved once against the skeleton and cached; unknown joint paths are ignored. The ankle's authored height above the clip's ground plane is preserved above the real terrain, corrections beyond 0.5 m are treated as non-ground and ignored, and a collinear (straight-leg) pose falls back to the chain's configurable bendDirection.
• Ground sampling defaults to a downward scene ray pick (octree picking) that rejects hits on the character itself or its descendants; games can override per entity with setFootIKGroundQuery (heightfield, navmesh, physics — anything).
• Runs after root motion and transitions, before pose composition, so it corrects the pose that will actually be displayed. Leg joints are assumed unit-scale.

How-to guide: docs/API/UsingFootIK.md. Single commit cherry-picked onto current develop (7d54789).

Testing

AnimationFootIKTests (10 tests, all passing):

• Two-bone solver: reaches a reachable target, clamps unreachable targets at full extension while pointing at the target, and solves a perfectly straight chain via the bend hint
• Ankle lands on raised ground with its authored height above the clip's ground plane preserved; ground below full extension clamps at full reach
• A ground sample far above the foot (beyond the 0.5 m limit) is rejected as non-ground; a missing ground sample leaves the pose untouched
• Off by default — and verified to never sample the ground while disabled; enable/disable round-trip; unresolvable chain paths are dropped without effect
• Placement tests drive the ground through a per-entity setFootIKGroundQuery injection, exercising the override path end to end

Root motion, inertialization, compiled-sampler, and policy suites still pass on this base. SwiftFormat lint clean.