nodeApi docs: derive the reference member index from the compiler, not a hand-kept table

Follow-up owned from review of #16792 (thread on `docs/node-api/reference.md:43`).

## Context

The reference's navigable table is hand-maintained and omits reachable public roots and members — `Comfy.system` / `SystemHandle.monitor()`, `playSound()`, `documentId()`, `onPromptSerialize`, `PromptInputProjection`, `PromptInputProjector`. The generated capability block names some omissions but gives no usage route.

## Decision

Ticket rather than hand-edit. Adding the missing rows fixes today's list and leaves the mechanism that let it drift. `scripts/node-api/gen_api_surface.mjs` already walks the type graph from the exported `Comfy` root to derive the conformance allowlist; the same walk can emit a member index for the reference, and `capabilityDoc.test.ts` already shows the pattern for failing when docs and code disagree.

## Done looks like

- A generated public-member index (root → member → type) embedded in `reference.md`, derived from the `Comfy` type graph
- A test that fails when the embedded index differs from the derived one, like the existing capability-list check
- Prose sections link into the index rather than maintaining their own partial member lists

Start with scripts/node-api/gen_api_surface.mjs and trace how it walks the exported Comfy type graph and produces the conformance allowlist. Then read docs/node-api/reference.md:43 and capabilityDoc.test.ts for the existing documentation-check pattern. Done means reference.md contains a generated root → member → type index, a test detects drift, and prose sections link to it instead of keeping partial lists.

Written by the indexing model from the issue text.