docs: add SDK trial stability comparison examples - #908

Problem

The comparison reference and CI tutorial still said generated SDK support for trial stability was unavailable after SDK #33 implemented the typed flag and response.

Change

• Add Python, TypeScript, and cURL comparison snippets using FI_API_KEY, FI_SECRET_KEY, and FI_BASE_URL.
• Name the generated Python RunComparisonRequest and existing experiment_runs.compare / experimentRuns.compare facades.
• Pin the draft backend/SDK prerequisites and preserve the distinction between test builds and published package/hosted availability.
• Add short saved-report examples that keep null statistics distinct from observed zero spread.
• Update the existing external-workflow and CI guide links; no new page, route, feature, or UI is added.

Metadata selector follow-up

The existing comparison reference and CI guide now document exact metadata_equals (20-key bound) and split_name selectors, selection counts, whole-identity semantics, and null/zero JSON preservation.

Validation

• npm run build: passed; Pagefind indexed 725 pages.
• npm run audit-links: zero broken navigation/content links; 29 existing orphan pages.
• Exact Python and TypeScript reference snippets were extracted from MDX and run using SDK prompt template sdk #33 against a temporary local fixture server. The server asserted both opt-in bodies and auth header injection, then returned the retained authenticated backend response fixture. Both printed unavailable statistics and zero spread correctly.
• Exact Python and TypeScript saved-report snippets were extracted and executed against the retained JSON response.
• git diff --check passed; the temporary dependency symlink was removed before staging.

The fixture-server check validates these exact snippets and generated SDK decoding. Earlier authenticated HTTP/PostgreSQL probes validate the SDK backend data path using deterministic synthetic scores. Neither establishes model/provider execution, live Observe ingestion, browser/UI qualification, hosted rollout, or package publication.

Stack: docs #904 -> docs #907 -> this PR. SDK dependency: futureagi-sdk #33, commit dde3ef590728c6b03332465c1218116cfe37f610. Backend contract: core #3483, commit 32ea09fa63d0ffa2e5178d1ede94f0f8b5796b7a.

Linear: TH-8587.