[RL-Kernel] Add linear_logp strict fallback checks

[inaniloquentee]

Summary

• add structured linear_logp unavailable reasons and strict-failure execution decisions
• validate strict-fast temperature, active masks, TP metadata, dtype/downcast, result shape, and train-time autograd connectivity
• keep non-strict native fallback observable, and pass loss masks through Megatron logprob paths for strict validation
• document new fallback reason code and strict-failure telemetry

Stack

• Depends on [RL-Kernel] Productionize linear_logp telemetry #27 / feature/issue20-linear-logp-telemetry-phase2
• Stacked on Phase 1 PRs already carried by [RL-Kernel] Productionize linear_logp telemetry #27

Tests

• py -3.13 -m compileall vime\backends\megatron_utils\rl_kernel.py vime\backends\megatron_utils\loss.py vime\backends\megatron_utils\model.py tests\test_rl_kernel_linear_logp_integration.py
• py -3.13 -m ruff check vime\backends\megatron_utils\rl_kernel.py vime\backends\megatron_utils\loss.py vime\backends\megatron_utils\model.py tests\test_rl_kernel_linear_logp_integration.py
• py -3.13 -m pytest tests\test_rl_kernel_operator_adapter.py tests\utils\test_rl_kernel_execution.py tests\test_rl_kernel_linear_logp_integration.py tests\test_megatron_argument_validation.py tests\test_cp_utils.py tests\test_loss_cp_invariance.py -q
• pre-commit run --all-files --show-diff-on-failure --color=always

Fixes #21

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