Type-check token definitions

[rvcas]

Builds on #138 (parser support). Replaces the parse-only token stubs with full type checking, mirroring the existing utxo machinery. Token codegen is intentionally left as a follow-up.

Builds on #138 — until that merges, this PR also includes its parser commits.

Behavior

A token definition is now checked end to end:

token MyToken {
    storage {
        indexed let mut supply: u32;
        let mut owner: u64;
    }
    mint fn mint() { supply = 1; }   // storage in scope; no explicit return type
    burn fn burn() { supply = 0; }
    impl Token {                      // built-in interface: attach/detach
        fn attach(to: Utxo) {}
        fn detach(source: Utxo) {}
    }
    fn helper() {}
}

Rules enforced: at least one mint fn, exactly one impl Token, mint may not declare a return type, and the impl Token block must match the built-in attach(Utxo) -> () / detach(Utxo) -> () signatures. Additional impl <UserAbi> blocks are allowed.

Changes

• Type model (types.rs): Type::TokenAny + Type::TokenNamed, parallel to Utxo, threaded through every exhaustive Type match (apply, substitute, occurs/free-vars, env, exhaustiveness, docs, to-wasm handle lowering).
• Typed AST (typed_ast.rs): TypedTokenDef { name, parts, ty } with TypedTokenPart + TypedTokenGlobal { indexed }.
• Type checking (infer.rs): register_token (opaque handle), Token annotation → TokenAny, a built-in Token ABI registered at construction so impl Token reuses check_abi_impl, a guard rejecting user-declared abi Token, and infer_token (structural pre-pass, storage scope, mint return-type rule, impl conformance) plus apply_token and type export.
• Errors: new E0052–E0055 with rendered messages and docs.
• Language server (document.rs): token symbols, hover, and type-annotation collection.
• Tests: 8 typecheck snapshot tests (valid token + each error path).

Note

Token is now a reserved type name like Utxo. The struct_param.star codegen fixture happened to define struct Token, so it was renamed to struct Item (snapshot regenerated).

Known parallels-to-utxo / deferred

• check_abi_impl does not enforce parameter arity (pre-existing utxo limitation).
• No token unify arms (utxo has none either; token-typed values don't flow through unification yet).
• Token codegen in starstream-to-wasm remains a "not yet supported" stub.

[SpaceManiac]

Seems like the right path