Remove watch feature flag — promote watch to always-on

[JoshuaChi]

Watch is production-ready. The watch feature flag adds complexity with no benefit: fallback stubs return unimplemented, all real deployments enable it, and optional Arc<WatchRegistry> spreads null-checks across the codebase.

Scope:

• Remove watch feature from all Cargo.toml files
• Collapse Option<Arc<WatchRegistry>> → Arc<WatchRegistry> (always present)
• Delete #[cfg(not(feature = "watch"))] stubs in grpc_raft_service.rs and embedded.rs
• Remove enabled: bool from WatchConfig
• Remove #[cfg(feature = "watch")] gates from all test files
• Update CHANGELOG

Dependency: Merge after #300.