## Why

The local debug SCUM fixture can be registered with a reduced manifest that omits `server.run.distribution`. That makes `scum-alpha` visible and manageable, but `POST /api/v1/server-instances/scum-alpha/run/generate` is denied even though the first-party SCUM manifest declares run distribution support.

## What Changes

- Register the local debug SCUM plugin with the run distribution, dependency, client-manager, bridge, and remote-access declarations from the first-party manifest.
- Extend local debug smoke proof to verify `generate-run` is available for `scum-alpha`.
- Extend local debug smoke proof to generate a Windows AMD64 run distribution for `scum-alpha`.
- Make deterministic run generation recover from an already-created package artifact instead of surfacing `duplicate_resource` to the UI.
- Make repeated run self-update dispatches return the existing update job when the artifact/checksum/idempotency key match.

## Impact

- Affected specs: `scum-run-distribution-smoke`
- Affected code: `scripts/local-debug-smoke.sh`, `platform/service/distributions.go`