Parser: preserve `let mut` / `&mut` mutability and `send`

[StreamDemon]

Summary

Several parsed tokens are discarded rather than represented:

• let mut x — the mut is dropped (let _mutable = …); Stmt::Let has no mutable flag.
• &mut in unary borrow expressions — mutability dropped (let _mutable = …); only op: "&" survives.
• send expr; — lowered to a plain Stmt::Expr, losing send semantics.

Part of the v0.6.x parser-completeness work (see crates/AGENTS.md "Implemented subset").

Spec

• §4 ownership/borrowing (& / &mut); §5.3 let bindings; §8 actors send (§8.2, §16 send_stmt).

Scope

• Add a mutable flag to Stmt::Let and thread mut through.
• Preserve &mut vs & on borrow expressions in the AST.
• Model send as its own statement/expression node rather than a bare Stmt::Expr.
• Unit tests asserting the preserved flags/nodes.

Notes

Low risk, mostly mechanical AST additions. Needed before any borrow-check or actor-semantics stage can consume the AST.