docs(api): document persisted run comparison - #902

Adds the draft API reference for POST /model-hub/experiment-runs/compare/, with exact request fields, authentication scope, response/evidence semantics, limits and success/failure cURL examples. The endpoint compares recorded runs; this page does not promise task execution, SDK methods, a new UI or a release verdict.

Stack: parent docs #900 (docs/TH-8551-dataset-version-reads); implementation depends on future-agi/future-agi#3472. Keep this draft until the API and deployment are qualified for publication.

Linear: https://linear.app/future-agi/issue/TH-8574 and https://linear.app/future-agi/issue/TH-8565
SDLC: https://github.com/future-agi/internal-docs/pull/70

Validation:

• Fields, bounds, enums and authentication behavior checked against the #3472 serializers/view and its 21 passing PostgreSQL API tests, including actual API-key viewer auth.
• Astro build passed: 1,184 pages in 23.67s.
• Navigation/content link audit passed: zero broken links; 29 existing orphan pages reported.
• Deleted-page check passed; no pages removed.
• Diff check passed.

The illustrative cURL commands use a caller-supplied test deployment and IDs; these exact commands were not run against a hosted service. No hosted or browser qualification is claimed. AI use: Codex authored and verified this change.