feat: add Manager-Based root-state reset contract

[TATP-233]

Summary

• add a typed BackendRootStateLayout contract for canonical xyz+wxyz qpos and world-linear/body-angular qvel columns
• resolve and cache floating-root layouts in MuJoCo, Motrix, and MJWarp; keep Drake fail-closed because its current runtime metadata does not prove body-to-free-root ownership
• expose read-only 13-D EntityData.default_root_state plus transaction-owned root state/pose/velocity writes
• preserve one SimBackend.set_state commit per reset while converting community world angular velocity with the staged quaternion

Relates #1072
Umbrella: #1042

Scope and size

• source-derived lines: 0
• mechanical NumPy conversion lines: 0
• UniLab-specific additions: 826 lines (implementation, diagnostics, and focused tests)
• modified existing files: 12
• deleted lines/files: 9 lines / 0 files
• no event sampling term, production task migration, Hydra, runner, IPC, or script change

Validation

• make test-all
  • ruff format/check: passed
  • mypy: passed
  • pyright: passed
  • pytest: 1949 passed, 26 skipped, 275 deselected, 1 xfailed
  • benchmark smoke: module 32/33 and script 33/34; only platform-optional MLX skipped
• focused root/backend/entity/reset tests: 97 passed, 3 skipped, 3 deselected
• real MuJoCo/Motrix conformance verifies body-frame generalized angular velocity reads back as the expected world-frame body velocity