feat(env): support named-keyframe Manager defaults

[TATP-233]

Summary

• add SceneCfg.default_keyframe_name as the optional Manager-Based default-state selector
• resolve and validate one detached read-only keyframe qpos snapshot during env initialization
• inject the same snapshot into Entity root/joint defaults and ResetStateTransaction
• preserve the unconfigured qpos0 path and existing lazy reset behavior
• fail closed for invalid selectors, missing keyframes/capabilities, and invalid qpos arrays

Scope accounting

• files: 5 modified
• additions: 369
• deletions: 24
• source: 145 additions / 24 deletions
• tests: 224 additions

Local validation

• focused Manager-Based env: 33 passed
• adjacent entity/reset/event regression: 58 passed
• ruff: passed
• mypy: passed
• pyright: passed
• make test-all: passed (2002 passed, 27 skipped, 276 deselected, 1 xfailed; benchmark module mode 32/33 and script mode 33/34, with one platform-optional MLX skip in each mode)

Per the #1042 child workflow, all gates ran locally on final head 55ce742; remote CI/Docs are intentionally not triggered.

Refs #1042
Closes #1087