fix(dispatch): keep deleted session projections dismissed

[bobleer]

Summary

• prevent stale in-flight status responses and incomplete observer projections from recreating deleted Dispatch sessions
• persist dismissal tombstones in an independent ledger so stale renderer snapshots cannot overwrite deletion state
• enforce a single active Dispatch observer per renderer and discard responses from observers that lost ownership
• keep the Dispatch composer entry hidden for the 0.2.15 release while retaining the implementation behind an explanatory comment
• add focused projection lifecycle diagnostics and regression coverage

Verification

• manually verified deleting the two residual Dispatch sessions
• stopped and restarted pnpm run desktop:dev repeatedly
• confirmed the deleted sessions no longer return after restart

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