Phase 2: Add linear_logp strict-mode checks and fallback tests

[inaniloquentee]

Background

Split from #6 Phase 2: Productionize linear_logp.

This issue covers strict-mode validation and fallback behavior for linear_logp. Valid configurations should run without fallback under strict fast mode, while unsupported configurations should either fall back with a structured reason or fail clearly under strict policy.

Scope

Add strict checks for:

• temperature compatibility
• active mask validity
• TP metadata completeness
• dtype/downcast contract
• unsupported CP redistribution
• entropy requests
• unsupported backend/dtype/hardware combinations
• missing backward saved-state support
• fallback behavior under --rlk-fast strict

Add tests for:

• --rlk-fast auto
• --rlk-fast strict
• native fallback path
• unsupported configuration with structured fallback reason
• strict policy failure when fallback is not allowed

Acceptance Criteria

• Validated configurations have fallback=0 under strict fast mode.
• Unsupported configurations fall back with a structured reason under non-strict policy.
• Unsupported configurations fail clearly under strict policy when fallback is not allowed.
• Tests cover both successful strict execution and expected fallback/failure cases.
• Logs explain every linear_logp backend decision.