docs(tools): count the union member as an edit, and re-scope the runtime guards

The 'adding a language is two table entries plus its renderer' checklist
predates the `CodeSdkLanguage` union and now contradicts the mechanism
sentence beside it: following it literally leaves the union untouched, which
is exactly the excess-property error that sentence promises. It is three
parallel edits, in the note's Decision and Consequences and in the
`SDK_RENDERERS` JSDoc.

Two guard descriptions still claimed work the compiler took over. The
Decision's 'the drift this guards against' now names the `satisfies` pins and
leaves the guards their reachable case, a mounted runtime reporting a language
neither table knows; `resolveFlavor`'s JSDoc drops 'keeps the table coupled to
SDK_RENDERERS' for the same reason. The Consequences said a half-added
language 'cannot arise' for the runtime guards — it can, one PR later at the
consumer's integration point, and never on this base; the claim is now about
timing rather than impossibility.

packages/core/tools/src/code-mode.ts

@@ -135,8 +135,10 @@ const RUN_CODE_DESCRIPTION_PARAM_DESCRIPTION
  * runtime is mounted — the static schema harvest (doc catalog), which never
  * reaches a model — so that path degrades to {@link TYPESCRIPT_FLAVOR}. A
  * mounted runtime whose language has no flavor entry fails loud, exactly as
- * `requireCodeRuntime` rejects it at assembly: this keeps the table coupled to
- * `SDK_RENDERERS` and never emits a wrong-language schema for a real runtime.
+ * `requireCodeRuntime` rejects it at assembly. Keeping this table in step with
+ * `SDK_RENDERERS` is the compiler's job ({@link CodeSdkLanguage}); what this
+ * guard owns is the runtime-supplied language neither table knows, which never
+ * yields a wrong-language schema for a real runtime.
  */
 function resolveFlavor(peekRuntime: () => CodeRuntime | undefined): RunCodeFlavor {
   const runtime = peekRuntime()