refactor: use backend materializer facade

[TATP-233]

Summary

• route training and interactive entrypoints through the existing unilab.base.backend facade
• repair the manip-loco benchmark import that referenced a nonexistent backend module
• preserve lazy optional-backend loading and add import-boundary regression coverage

Scope

• Parent umbrella: Work: 代码清理审计 umbrella：边界契约/结构契约/死代码/重复实现/腐化代码结论汇总(只审计) #983
• Resolves child issue Cleanup: 训练与入口层统一使用 backend 公共 materializer facade #992 after this PR is merged into dev/issue-983-code-cleanup
• Integration target: dev/issue-983-code-cleanup only; no change targets main

Validation

• PYTHONPATH="$PWD/src" UV_NO_SYNC=1 make test-all
• pytest: 1675 passed, 27 skipped, 273 deselected, 1 xfailed
• benchmark smoke: 33/34 module imports and 34/35 script imports; only platform-optional MLX skipped
• pyright: 0 errors
• exact validated head: de8364668e6a6cafc00f581dcbb652c841578d37