docs: document metadata comparison selectors - #909

Problem

The comparison API and CI tutorial did not explain how to inspect one metadata/split cohort while preserving complete case/trial identity and missing evidence.

Change

• Document optional metadata_equals (exact JSON equality, max 20 keys) and split_name selectors on the existing comparison endpoint.
• Explain omission behavior, one-sided matching, whole (example_id, trial_number) identity selection, and the selection receipt.
• Add exact Python, TypeScript, and cURL snippets using SDK updated- observe, prompt sdk #34 and the core #3490 contract.
• Document JSON null, 0, false, arrays, and object preservation and the draft availability boundary.
• Update the existing CI release-gate tutorial to show cohort-scoped gating and selection receipt checks.

Validation

• npm ci
• npx astro build — 1,190 pages built successfully
• npm run audit-links — 0 broken navigation/content links (29 pre-existing orphan pages)
• git diff --check
• Synthetic JSON fixture verified cURL selector payload and nullable/zero values.
• SDK compatibility evidence: Python lifecycle tests 34 passed; TypeScript lifecycle tests 20 passed on SDK updated- observe, prompt sdk #34 (6cc03421cb).

Stack: docs #908 (0c1f2b31) -> this PR. Core #3490 (e0e40ac1ee), SDK #34 (6cc03421cb). Linear: TH-8593.

This is a draft. It does not claim published SDK packages, hosted availability, UI/Figma approval, model/provider execution, Observe trace linkage, or deployment/release qualification.