fix(motion): handle adaptive reset zero-floor cold start

[TATP-233]

Summary

• Add a uniform cold-start fallback when adaptive reset has zero failure statistics and adaptive_uniform_ratio=0.
• Add a regression test covering deterministic valid frame sampling and maximum entropy at cold start.

Validation

• uv run pytest tests/envs/test_motion_loader.py -q — 10 passed
• make check — passed
• make test — 1569 passed, 26 skipped, 606 deselected
• make test-all — passed

This prevents 0 / 0 normalization from producing NaN sampling probabilities during the first reset.