feat(base): add NumPy entity facade

[TATP-233]

Summary

• add a base-owned EntityCfg / EntityScene / EntityData facade over the existing public SimBackend contract
• materialize explicit joint/body/geom/site/actuator partitions once, cache local-to-backend mappings, and expose NumPy root/joint/body state
• preserve pinned mjlab SceneEntityCfg regex/order/consistency semantics while rejecting invalid IDs and unsupported namespaces with entity/backend context
• attach manager/term/parameter context to selector failures without coupling managers to backends or NpEnv

Boundaries

• no new SimBackend method or backend-private/model access
• no NpEnv lifecycle, production task, Hydra script, runner, learner, or IPC integration
• no XML/asset access in the facade and no name resolution on repeated state reads
• tendon/camera/light/material/texture/pair remain explicit unsupported capabilities

Change accounting

• source-derived: 55 lines for pinned mjlab-compatible regex matching, with path/commit/Apache-2.0 provenance
• mechanical adaptation: 110 added / 11 removed lines expanding the standalone manager entity protocol
• UniLab-specific facade, validation, error context, and tests: 969 added / 6 removed lines
• modified existing: 4 files, +148 / -17 lines
• created: 2 files, 986 lines
• deleted existing files: none
• total: 6 files, +1,134 / -17; within the ≤25 file / ≤1,500-line child budget

Validation

• focused manager/entity suite: 45 passed
• config + representative backend regression: 211 passed
• make test-all: 1,814 passed, 24 skipped, 1 xfailed; benchmark import smoke passed 65/67 with two platform-optional mlx skips
• ruff, mypy, and pyright: passed

Implements #1047
Umbrella: #1042