calibration: wire probeFunctionalSurface into the live provider/entry gate for openapi/subnet-api/sse kinds

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Problem

src/review/content-lane/registry-logic.ts:636's probeFunctionalSurface is a "best-effort, truncation-tolerant check that a fetched body actually serves the surface its kind claims" (openapi schema, JSON API, SSE stream) -- fully implemented and tested (11+ cases in test/unit/content-lane-registry-logic.test.ts). Like computeGrounding (see the sibling issue), it has zero callers outside its test file; assessSurfaceEntry never fetches the surface url's content before merging.

Proposal

Same shape as the sibling grounding-check issue -- fetch the surface url's content-type/body for FUNCTIONAL_KINDS (openapi, subnet-api, sse) entries and feed it through probeFunctionalSurface, holding/closing on served: false. Likely should be scoped/designed together with the grounding-check issue since both need the same "fetch source content before merge" plumbing.

Deliverables

• A maintainer decision on whether/when to wire live functional-surface probing into the merge gate
• If approved: a design spec covering fetch plumbing shared with the grounding-check issue, and what served: false does to the gate decision

Links & Resources

• src/review/content-lane/registry-logic.ts:636
• test/unit/content-lane-registry-logic.test.ts

Boundaries

This changes what's verified before a subnet-registry surface auto-merges -- gate-authority change requiring design review. Do not unlock for contributors without an explicit maintainer decision. Should be scoped together with the sibling grounding-check issue given shared fetch-plumbing needs.

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JSO-6188