Wire repository_dispatch from Fallout preview.yml

[ChrisonSimtian]

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".