refactor(sacp): extract role-agnostic MatchMessage from MatchMessageFrom

- Add new MatchMessage struct for pure parsing without role awareness
- MatchMessage provides if_request, if_notification, if_message methods
- MatchMessageFrom now delegates parsing to MatchMessage
- MatchMessageFrom applies endpoint transforms before delegating
- Both types share the same fluent API pattern

This separation allows simpler parsing when endpoint transforms
aren't needed, while keeping role-aware matching for proxy scenarios.

Author: nikomatsakis

src/sacp/src/util.rs

@@ -1,7 +1,7 @@
 // Types re-exported from crate root
 
 mod typed;
-pub use typed::{MatchMessageFrom, TypeNotification};
+pub use typed::{MatchMessage, MatchMessageFrom, TypeNotification};
 
 /// Cast from `N` to `M` by serializing/deserialization to/from JSON.
 pub fn json_cast<N, M>(params: N) -> Result<M, crate::Error>