fix(lifecycle): share session end with local core supervision - #6119

Summary

Passes the session-end intent from #6117 directly into the local core manager. While a session end is pending, the local core is not restarted automatically, and on Windows a stop kills the existing process tree without launching the --nyanpasu-process-shutdown Ctrl+C helper. Revoking the intent (a cancelled shutdown) restores normal supervision and graceful stops.

Stacks on #6117. Depends on libnyanpasu/nyanpasu-runtime#470 (stacked on #469) and libnyanpasu/nyanpasu-utils#218. Refs #6054.

Changes

• Pin backend/nyanpasu-runtime to 8250238 (R1, which pins utils a3ec581).
• SessionEndSource/SessionEnding keep their public bool API but are backed by one utils ProcessActivitySource, so the Windows hook's write and the supervisor's read are the same input, with no forwarding task. The reader still reports that its source was dropped.
• local_host::build takes the session reader explicitly and passes its process activity to the core manager. setup.rs provides it from the composition root.
• fake-core: the existing start-barrier error now carries the PID and is published on exit, which the regression test uses as a deterministic start record.

Verification

• revoking_session_restores_normal_supervision drives the real fake-core through CoreControl. While pending, the first attempt stays the recorded process. After revocation, exactly two retries run. It failed with the activity wiring removed, and it passed 5 consecutive runs.
• Passing: clash-nyanpasu tests (189), clippy, rustfmt, lint:architecture-ledger, lint:backend-boundaries and the scripts tests.
• Known environment-only failures: nyanpasu-core's hwid test and nyanpasu-platform's proxy-port test.
• The Windows tree kill and the zero-helper behavior still need verification on Windows.

Merge order

Merge after #218, #469 and #470. If any of them is squash-merged, re-pin the submodule to the merged commits first.