#4313 refactor(capture-kit): state each snapshot source's circuit scope

Summary

Applies the optional items from the #4308 review. Part of #4205. No behavior change.

1. Circuit scope union. SnapshotSourceChannel.circuit is { scope: 'route' } or { scope: 'runner-session'; sessionChanged() }. A runner-session channel must supply sessionChanged, and a route channel cannot declare one. The runner bridge is runner-session; the local guest channel is route. A new test proves the local route keeps a retired generation's circuit across a runner session change.
2. Shared layering helper. scripts/layering/executable-policies.ts evaluates every executablePolicies entry. It replaces platformImportsFromRouter and daemonImportsFromSnapshotFacet.
3. JSDoc. RunnerSessionCircuit states that only a runner-session circuit can set sessionDisabled. A duplicate comment in lifecycle.fixtures.ts is removed.

Validation

At 69621eb183 (base 0e7df74a1a):

• pnpm check:affected --run --head 69621eb183: all runnable checks passed, exactCommit=true (41 passed).
• Vitest capture-kit + platform-apple, node --test scripts/layering/*.test.ts, pnpm typecheck, pnpm check:layering: pass.
• Mutation controls (reverted): local channel as runner-session → the local-route test fails. Runner channel as route → 4 session-circuit tests fail. Helper filter or policy roots broken → the helper tests fail.

No device run: route behavior is unchanged.

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