v1.21.0 — Mechanism simulation upgrade

Turns the motion sim into a real mechanism tool — addresses a full P0–P2 backlog from an external knitting-machine build. Backward compatible: existing sim blocks run unchanged.

Kinematic authoring

• pose-tracks — drive a body with one time-varying transform (closed-loop escape hatch)
• revolute unit: "deg"; new profiles firstOrder (dead-time + τ), slew, servo; keyframe interpolation (linear/smoothstep/cubic)
• markers — named points whose world path the sim reports

Constraint solver

• analytic four-bar / slider-crank / gear solver — a four-bar coupler is ONE linkages entry, loop closure solved automatically
• contact intervals (re-collision on the return stroke is captured)

Dynamics (Rapier)

• CCD on by default; realistic density for mm-scale stability
• real joints + PD motors on dynamic bodies — a cam can drive a pinned follower; per-body collider choice; solver-iterations exposed

Discoverability & self-test

• zod validation with actionable errors; get_api_reference("simulation") with units explicit
• assertions channel (pass/fail); sim can be a function of params; richer report (orientation, markers, contact windows)

UI

• the timeline is now a first-class extension panel (toolbar Sim button + docked play/scrub/speed + collision & assertion log)

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