host: add supervisor-gated clean core.restart RPC

Return success before scheduling the existing shutdown path, coalesce pending
requests, and mark #loop invocations with SAUSAGE_LOOP=1. Keep core.exit
compatibility and document explicit per-call approval for all agents.

Card: SAU-134

flake.nix

@@ -80,7 +80,7 @@
         # `nix run .#loop -- <world>` runs the newest `sausage run <world>`
         # (without a folder: bare `sausage`, i.e. ./world, created if needed)
         # from the public repo, again and again: after a crash or a call of
-        # `core.exit` it refreshes the flake and starts the host again, so
+        # `core.restart` (exit code 75) it refreshes the flake and starts the host again, so
         # the world updates itself. `quit` (exit code 0) ends the loop.
         # SAUSAGE_FLAKE overrides where it fetches from (any flake URL,
         # e.g. a fork or a local checkout).
@@ -90,7 +90,7 @@
           # No folder: bare `sausage` (creates ./world first if needed).
           if [ $# -gt 0 ]; then set -- run "$@"; fi
           while true; do
-            nix run --refresh "$url#default" -- "$@"
+            SAUSAGE_LOOP=1 nix run --refresh "$url#default" -- "$@"
             code=$?
             if [ "$code" -eq 0 ]; then exit 0; fi
             echo "sausage-loop: sausage exited with $code, starting the newest version in 2 s …" >&2