perf(dispatch): reuse snapshots and resume observers

[bobleer]

Summary

• reuse cached workspace archives when metadata changes but the selected file tree remains byte-identical
• persist the rendered observer transcript together with its cursor so Dispatch projections can resume after restart without replaying the complete event log
• report target model readiness from actual model configuration parity instead of comparing an unrelated configuration id
• keep the composer Dispatch target picker hidden behind the existing release gate while preserving completed-result review

Verification

Automated checks were not rerun during this final iteration at the request of the verifier.