docs: Document POST /v1/systemone: the native System One API, served by regular HTTP workers - #153

Reviewed SMG source: a29c64d4323dbf21950e2822cb099af04f6fd901.

The PR adds exactly one concern — the /v1/systemone endpoint — as one row plus three short paragraphs on the allowlisted canonical page (reference/api/extensions.md), 8 added lines, matching the page's /v1/interactions row style. Every behavioral claim verifies against the checkout at a29c64d4: route registered among protected inference routes behind auth_middleware (server.rs:1117, :1158-1161); only regular HTTP workers serve it with 501 systemone_not_supported when a model has no HttpRegular-pool worker in IGW mode (gateway.rs:711-724) or when the deployment's router is not HTTP-regular (routers/mod.rs:218-229); unknown/wildcard models answer 404 model_not_found in both the IGW path (gateway.rs:702-715) and direct HTTP router placement (http/router.rs:575); ValidatedJson answers 400 for state/questions shape failures and for empty questions or empty score criteria (validated.rs:364,374; systemone.rs Validate impl); unknown fields are forwarded via serde(flatten) plus serialize_request_body_preserving_fields (http/router.rs:659-665); and the worker's status, body, and headers (minus hop-by-hop) are relayed unchanged for success and error statuses (forward_worker_response, header_utils.rs:118-130), with is_stream() pinned false. Release framing: the v1.12.0 tag exists (packed-refs:61), the source commit a7e28280 (2026-10-07, pipeline-supplied patch) predates the claimed 2026-10-08 v1.12.0 bump, current main is versioned 1.12.0 with release workflows defaulting to tag v1.12.0, and merged docs already treat v1.12.0 as the latest release with later changes marked "newer than v1.12.0"; limitation: with Read/Glob/Grep only, the tag-contains-commit ancestry could not be re-executed in git, but all readable evidence is consistent and nothing contradicts it. The release note is its own short standalone paragraph, no new page was created, no PR/commit/code references appear in the added prose, and the deferred /v1/decisions row was correctly not added. Related pages are consistent: metrics.md:828 already lists the systemone endpoint label, and the buffer-vs-stream enumerations (metrics.md:418, request-streaming.md:65) correctly exclude /v1/systemone per the middleware allowlist (http/router.rs:2106-2114). The feedback section contains no threads, comments, or failed checks, so addressed_threads is empty.