Add armed plugin-originated file watches across VFS mutations

Register core.watch before its armed notification, coalesce later changes
without losing post-notification writes, and clean up on caller death.
Observe mounted layers and open file handles, including descriptor/stream
writes, truncation, both rename paths and deletion. Track cancelled
mutations through process idle and retain visible paths across renames.
Document the protocol, expose native core discovery, and exercise watches
through SDK callbacks and raw WASI filesystem operations.

Card: SAU-359

cli/tests/host/main.rs

@@ -7,3 +7,4 @@ mod common;
 mod console;
 mod core;
 mod sdk;
+mod watches;