[digitalrisedorset]

Title

feat: add Beach Finder service and widget

Description

Summary

• Add an Express service that accepts a UK postcode and search radius, then returns nearby beaches with coordinates and straight-line distances.
• Use Postcodes.io for postcode lookup and OpenStreetMap Overpass for beach data.
• Add a runtime-shadow widget with postcode and radius inputs, loading, empty, success, and error states, plus OpenStreetMap links and attribution.
• Update the canonical runtime widget templates and structure validator to support required, widget-specific runtime manifests.
• Add Rollup to the widget template dependencies so generated widgets build successfully.

Verification

• Service tests, typecheck, build, lint, and live status request passed.
• Widget tests, build, lint, JSON schema checks, and structure validation passed.
• Platform MCP typecheck and tests passed (22/22).
• Lizard check passed; no functions exceed CCN 10.
• Playwright UI spec added but not run; it requires the host page.

No deployment or merge performed.