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/lib.rs

@@ -5,6 +5,7 @@ mod components;
 pub mod console;
 mod core;
 mod execution;
+mod filesystem;
 mod graph;
 mod host;
 mod journal;
@@ -14,6 +15,8 @@ mod process;
 mod routing;
 pub mod runtime;
 mod services;
+mod upgrades;
+mod watcher;
 pub mod world;
 pub use host::Host;
 pub use native::{NativeContext, NativeFuture, NativePlugin};