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

server/test/eventsRoles.test.js

@@ -134,6 +134,9 @@ const forbidden = async (method, path, role) => (await dispatch(method, path, ro
 const SURFACE = [
   // Reads: staff-wide, the tier gate and nothing added.
   ['GET', '/events/catalog', ['admin', 'editor', 'moderator']],
+  // Phase 7. Authoring data, so staff-wide like the catalog it belongs to: an
+  // editor who may write the step must be able to see which values it accepts.
+  ['GET', '/events/catalog/options/core.options.legs', ['admin', 'editor', 'moderator']],
   ['GET', '/events/series', ['admin', 'editor', 'moderator']],
   ['GET', '/events/calendar', ['admin', 'editor', 'moderator']],
   ['GET', '/events/runs', ['admin', 'editor', 'moderator']],