docs: plan hot path optimizations

[ferueda]

Summary

• add four code-backed program plans for everyday queries, indexing, doctor/maintenance, and verified bounded session reads
• define ordered delivery slices, provider-free measures, exact correctness checks, privacy boundaries, crash/recovery coverage, and evidence-based stop gates
• replace the completed digest-guard executor plan in the active-plan index

Why

The next performance work spans several independent hot paths. Writing the plans first makes the intended algorithms, integrity requirements, measurement seams, and promotion gates reviewable before implementation begins.

Impact

Documentation and planning only. Runtime behavior, storage, CLI contracts, and provider access are unchanged.

Validation

• pnpm check
• 121 test files and 1,271 tests passed
• three independent design reviews completed; final revised plans received PASS verdicts