feat(events): open the event contract to modules (Phase 7)

MODULE_API 1.10.0. Four names forwarded on the module-facing `api` --
registerEventActions, registerEventBudgets, registerEventLeases and
registerEventOptionSources -- one new route, and one rule made real: a
`cost()` naming a dimension no module declared is refused.

Only one of the four is new machinery. The action registry has staged
core's three actions on every boot since Phase 1; what it never had was a
way in, because loader.js builds its own `api` facade and had no method
that delegated to it. So the registry a module now reaches is one that has
been exercised on every boot for six phases.

Four decisions, settled 2026-09-03, all as recommended:

- Option sources are their own registration, modelled on registerAudiences,
  because a catalog has more than one consumer.
- An undeclared dimension is refused -- at save, at the dry run and at
  dispatch -- with its own code, because the fix is a module's declaration
  and not a deployment's cap.
- A lease is declared here and acquired by nothing; the ledger is Phase 8.
- Core registers core.options.legs, so an announce leg is a dropdown rather
  than the free-text box whose typo Phase 6's walk caught mid-run.

Proved with a throwaway module through the real loader, not with module-uo:
eventModuleContract.test.js writes a module to a real directory and lets the
loader scan it, covering all five envelope failure shapes, verify: true, the
four id spaces and dormancy on uninstall.

The live walk found the one defect nothing else could: the option-source
loader wrote its "already asked?" guard inside a setState updater and read
it on the next line, so the request was never made and the field sat on
"Reading the list..." for ever. It is a useRef now.

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T6t8mrAWhZU5vnyYgZTMtL

client/src/api/client.js

@@ -486,6 +486,13 @@ export const api = {
     archiveEvent: (id) => req(`/admin/events/${id}`, { method: 'DELETE' }),
     listEventVersions: (id) => req(`/admin/events/${id}/versions`),
     eventCatalog: () => req('/admin/events/catalog'),
+    // Phase 7. The values behind a param's `source` — resolved by the module that
+    // registered the source, on a request of its own rather than inside the
+    // catalog, because a source can be slow or down and must not take the whole
+    // editor with it. A refusal comes back 200 with `ok: false`, so this never
+    // throws for the case the screen is meant to render: the field degrades to
+    // free text with the reason beside it.
+    eventOptions: (sourceId) => req(`/admin/events/catalog/options/${encodeURIComponent(sourceId)}`),
     // Phase 6. The dry run is admin+editor: it dispatches nothing, and the author
     // who wrote the definition is who should be able to price it against the caps
     // before asking an admin to publish it. A report with findings comes back 200