Unify execution interfaces to new one, deprecate old one

[socksy]

Standardising on one execution interface. Renames ExecutionBackend/ExecutionHandle back to Backend/App (the old names) and adds default method impls so backends don't stub what they don't use. LocalApp implements App directly now; SubprocessHandle sticks around as a thin wrapper because auto_cleanup's temp-dir lifecycle has to hang off something.

Cancellation used to ride in as exit code -1, which conflated it with IO errors and signal kills. execute_local_app now sends a Result<Status, Error> and Status grows a Cancelled variant. Hopefully this lets the control plane drop the "preserve Cancelled if last_known_status was Cancelled" heuristic.

Cancelled is a hard break for exhaustive matches — one-line arm needed in tower-runner when it relinks, tracked out of band. The trait rename is kept deprecated-but-defined (not aliased) so downstream impls compile with a guiding message rather than an unresolved-import error. Root-level tower_runtime::App is also deprecated in favour of execution::App.

[sammuti]

Note this would require a change in the runner side for the k8s impl. I propose just keeping the new name rather than renaming since I believe it's better name than "App"

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[bradhe]

I think the main thrust of this PR is to standardize the interfaces slightly better than they are now. I do think that's valid, it just needs to be modernized a bit. I don't have strong opinions about how that should come together however. I'd be open to keeping this around and reintegrating the latest into it.