core: sausage-vfs per process (/, /proc, /self, /run), activation, graph, watcher, upgrades

- /proc is a core layer: core/ (PROTOCOL.md, graph.json, graph.dot,
  locks.json, plugins/<name>/discover.json and client.rs) and one folder
  per process (info.json, log, and its /self files).
- Locks are released only after the task ended, the store dropped, and
  in-flight file operations returned.
- core.activate and core.revert with copies, discovery, compatibility
  checks of imports and consumers, groups, force. An admission gate
  makes the snapshot of old processes complete.
- Graph of discover documents by hash, refreshed by a file watcher.
- Upgrades: grace periods (x-sausage-grace), stop, service restarts,
  rpc.retired.
- SDK plugins in the real core (cli/tests/sdk.rs).

cli/src/core.rs

@@ -31,7 +31,19 @@ impl Host {
                 Ok(Value::Null)
             }
             "kill" => {
-                self.kill(required_str(&p, "pid")?);
+                let pid = required_str(&p, "pid")?;
+                let ended = self
+                    .0
+                    .state
+                    .lock()
+                    .unwrap()
+                    .processes
+                    .get(pid)
+                    .map(|p| p.ended.subscribe());
+                self.kill(pid);
+                if let Some(mut ended) = ended {
+                    let _ = ended.wait_for(|ended| *ended).await;
+                }
                 Ok(json!({}))
             }
             "stop" => {