#1536 Add --server support to model cancel (server-side run cancellation)

Problem

model cancel was excluded from #1531 because it requires a fundamentally different server-side approach. The local implementation kills processes by PID via killProcessTree() — this cannot work over --server since the client has no access to server PIDs.

Proposed Solution

Add a run.cancel protocol message type (or reuse the existing cancel message) that cancels a running model execution by name/ID on the server side. The server handler should:

1. Look up the active run by model name or run ID
2. Abort the run via the RunCancelRegistry or AbortController
3. Return confirmation of cancellation

The existing cancel protocol message cancels an in-flight WebSocket request by ID — this is different from cancelling a named model run.

Context

Filed as a follow-up from #1531 / #2080.

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Automoved by swampadmin from https://github.com/swamp-club/swamp/issues/2081