Wire repository_dispatch from Fallout preview.yml

**Problem.** Canary only runs nightly + on manual dispatch. It should fire immediately on every `-preview` publish for fast feedback.

**Outcome.** `Fallout/.github/workflows/preview.yml` POSTs a `repository_dispatch` (`event_type=fallout-preview-published`) to this repo after a successful publish. The listener already exists in `canary.yml`.

**Acceptance criteria.**
- [ ] `CANARY_DISPATCH_TOKEN` secret (dispatch scope on `fallout.canary`) added to the Fallout repo.
- [ ] Dispatch step added to `preview.yml` (snippet in `docs/DESIGN.md`) via a PR labelled `target/2026`.
- [ ] A `-preview` publish triggers a canary run automatically.

See `docs/DESIGN.md` → "Wiring the dispatch".

Read Fallout/.github/workflows/preview.yml, the existing listener in canary.yml, and the "Wiring the dispatch" section of docs/DESIGN.md. Add the post-publish dispatch and ensure the CANARY_DISPATCH_TOKEN secret is configured in the Fallout repository, using a PR labelled target/2026. Done means a -preview publish automatically starts a canary run.

Written by the indexing model from the issue text.