Run world service policy in WASI init with pollable process observation

host/wit/process.wit

@@ -3,6 +3,14 @@ package sausage:process@0.1.0;
 /// Programs share one world. These resources provide lifecycle and bounded stdio transport.
 interface host {
     use wasi:io/streams@0.2.12.{input-stream, output-stream};
+    use wasi:io/poll@0.2.12.{pollable};
+    enum change-kind { created, modified, removed, overflow }
+    record change { path: string, kind: change-kind }
+    resource watcher {
+        constructor(paths: list<string>);
+        subscribe: func() -> pollable;
+        changes: func() -> list<change>;
+    }
     enum stdio { pipe, inherit, null }
     record request {
         program: string,
@@ -21,6 +29,7 @@ interface host {
         stdin: func() -> output-stream;
         stdout: func() -> input-stream;
         stderr: func() -> input-stream;
+        subscribe: func() -> pollable;
         status: func() -> option<s32>;
         wait: func() -> s32;
         kill: func();