Tests: Golden motion fixtures (JSON)

[flyingrobots]

Add JSON golden fixtures and minimal harness for motion rule; ensure bytes round-trip and positions match expected component-wise using to_bits() equality.

[flyingrobots]

Closing: PR-01 (#90) delivered golden motion fixtures and harness; PR-06 (#97) expanded negative cases (NaN/Inf, invalid size). Bytes round‑trip and to_bits() comparisons are asserted. Future fixture additions can be tracked separately.