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/Cargo.toml

@@ -26,3 +26,9 @@ wasmtime-wasi-http = "49"
 uuid = { version = "1", features = ["v7"] }
 sha2 = "0.10"
 rusqlite = { version = "0.38", features = ["bundled"] }
+sausage-vfs = { version = "0.1.0", path = "../vfs" }
+sausage-interface = { version = "0.1.0", path = "../interface" }
+notify = "8.2.0"
+
+[dev-dependencies]
+sausage-interface = { path = "../interface" }