feat: add gated public archive query

[SemeAIPletinnya]

What changed

• adds a packaged, hash-locked PUBLIC-only evidence index
• adds deterministic retrieval and candidate construction without an external model or network ingestion
• adds POST /v0/archive/query, which sends every candidate through the existing Gate
• returns the exact candidate only for SHOW / PROCEED; REVIEW and BLOCK return releasedAnswer: null
• preserves legacy receipt_id while keeping decision and execution receipt identities distinct
• persists an allowlisted candidate/evidence trace in the decision receipt,
  explicitly marked as metadata rather than Gate authority
• adds endpoint-specific public rate metadata while retaining the non-persistent shared abuse guard
• adds adversarial SILENCE, prompt-injection-as-data, CORS, rate-limit, no-evidence, visibility, health-contract, and HTTP tests
• adds a six-task local pilot dry run explicitly marked DRY_RUN_NOT_HUMAN_EVALUATION

Why

This implements roadmap phases 3–4 and the dependency-valid public parts of
phases 9–10 without changing SaC/PoR semantics. Retrieval is not truth,
generation is not release authority, and no fallback or warning is substituted
after a Gate hold.

User/developer impact

A public Axiom client can query admitted public evidence through a deterministic
local path. Private archive, raw archive, online ingestion, external LLM calls,
conversation persistence, and downstream execution remain off.

Validation

• pytest: 104 passed
• contract check: passed
• benchmark: 100/100
• public pilot dry run: 6/6 tasks passed; human pilot remains held
• index SHA-256 matches the public-site mirror

Protected D:\SemeAi\silence-as-control was not changed.