feat(registry): support callable env factories

[TATP-233]

Summary

• define one EnvFactory contract for existing class constructors and plain function factories
• make register_env / env preserve factory identity and make registry.make() pass the materialized cfg, num_envs, and selected backend_type
• fail closed at the Registry boundary when a callable returns a non-ABEnv value; preserve config materialization, override, validation, and backend-selection order

Refs #1096
Umbrella #1042
Architecture: ADR-0006, ADR-0004

Scope

• 4 files
• 128 additions / 24 deletions / 104 net lines
• no Manager-Based backend adapter, production Go2 registration, Hydra, training script, runner, backend, or IPC changes
• existing class registrations and backend behavior remain compatible across MuJoCo, MJWarp, Motrix, and Drake

Validation

• make test-all: 2023 passed, 27 skipped, 276 deselected, 1 xfailed
• ruff, mypy, pyright, coverage suite, and benchmark smoke passed
• remote CI intentionally skipped per Roadmap: 将 UniLab 迁移为社区通用的 Manager-Based API #1042