feat(docs-gate): close the runtime-services stability label vocabulary (#9751) (#10008)

`stable` / `experimental` was the whole vocabulary, written out by hand in two
places that nothing compared -- the "Stability Legend" table in
`runtime-services/index.mdx` and the "Stability Labels" bullets in
`versioning.mdx` -- with no enum in code behind either. Measured on this tree
before this change: `audit-service.mdx` declaring `- **Stability:** `beta``,
with the Current Matrix row and the kernel/index.mdx row faithfully repeating
`beta`, was green while neither legend defined `beta` anywhere; a `beta` row
added to one legend alone was green too.

The vocabulary now has ONE definition, `STABILITY_VOCABULARY` in the existing
gate. Both legends are held to it by exact ordered EQUALITY -- not membership,
which passes one legend dropping a row the other keeps -- and every page's label
is drawn from it. The two tables are deliberately left to check 7, which already
holds them to the page, so an out-of-vocabulary label reports once, against the
file where the label was chosen.

No new gate: this is checks 9-10 of `check-runtime-services-index.mjs`, per the
ruling that a second recognizer on one surface is how two recognizers end up
disagreeing.


Claude-Session: https://claude.ai/code/session_01XqDQYVU5smx29ts9pAErja

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-steve

content/docs/kernel/runtime-services/index.mdx

@@ -41,6 +41,13 @@ This chapter documents the runtime `services.*` APIs used in hook/action/flow/pl
 | `stable` | Backward-compatible within a major version |
 | `experimental` | API is available but may change in minor releases |
 
+These two labels are the whole vocabulary. The same list is written out again as
+[Stability Labels](/docs/kernel/runtime-services/versioning), and
+`scripts/check-runtime-services-index.mjs` holds both legends — and every page's
+**Stability** bullet — to one definition, so a page cannot publish a label neither legend
+defines. Adding a label means extending that definition in the same change that says what
+it promises, in both places.
+
 ## Source of Truth
 
 Each page links the canonical TypeScript source used to derive signatures.

content/docs/kernel/runtime-services/versioning.mdx

@@ -10,6 +10,12 @@ description: Stability labels and how to track runtime service API breaking chan
 - `stable`: backward-compatible within the current major version.
 - `experimental`: can change in minor versions; verify release notes before upgrades.
 
+These two labels are the whole vocabulary. The same list is written out again as the chapter's
+[Stability Legend](/docs/kernel/runtime-services), and
+`scripts/check-runtime-services-index.mjs` holds both legends — and every page's **Stability**
+bullet — to one definition, so the two lists cannot drift apart. Adding a label means extending
+that definition in the same change that says what it promises, in both places.
+
 ## Breaking-Change Tracking
 
 When a runtime service API changes in a breaking way: