docs: add case-level release gate workflow - #904

Adds draft case-level release-gate and dataset portability API references/tutorials on top of docs #903.

Scope:

• document the optional release_gate policy on the existing comparison read;
• explain complete-plan and fail-closed evidence semantics, regression allowance, critical cases, score direction, deterministic blockers, and HTTP 200 failed verdicts;
• add a standard-library Python CI caller that saves the report and exits 0/1/2 for pass/fail/request errors;
• add the SDK JSONL dataset-version portability guide, including stable IDs, server hashes, checksum, limits and explicit legacy-row policy;
• update API and guide navigation.

Validation:

• link audit: 729 pages, zero broken navigation/content links; 29 existing orphan pages;
• Astro build: 1,189 pages in 17.30 seconds;
• Python compile/Ruff/diff checks and eight CLI regression tests pass;
• deterministic local HTTP smoke scenarios: regression exit 1, allowance pass exit 0, critical regression exit 1, incomplete plan exit 1, malformed policy exit 2;
• SDK portability tests and HTTP roundtrips are recorded on SDK Feature/revamp #32, not claimed as docs-hosted execution;
• no hosted, model-backed, worker, Observe, browser, SDK release, or deployment qualification is claimed.