docs(launcher): design stale sync socket guard

docs/plans/2026-07-09-sync-stale-socket-guard-design.md

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+# Sync Stale Socket Guard Design
+
+## Problem
+
+The sync quiescence guard treats any configured mpv socket path as a running
+session when the filesystem entry exists. Unix socket files can remain after
+mpv exits, so a stale `/tmp/subminer-socket` blocks the remote merge even when
+nothing is listening.
+
+## Design
+
+Reuse the launcher's existing Unix socket connection check. The sync guard will
+block only when it can connect to the configured socket. Missing, stale, or
+otherwise unreachable socket paths will not block sync, and sync will not
+delete them.
+
+Because socket connection checks are asynchronous, the sync dispatch, merge
+mode, host mode, and quiescence checks will become async. The already-async
+launcher entrypoint will await sync dispatch. Snapshot, transfer, merge,
+cleanup, and error behavior otherwise remain unchanged.
+
+## Validation
+
+Add focused tests proving a stale socket is ignored and a live socket remains a
+hard failure. Re-run sync command and socket tests, type checking, launcher
+tests, docs checks, and the SubMiner launcher/docs verification lanes.