feat(types): function types as a third flavour (type: fn A, ...: R)

[kollhof]

What

A fn A, ...: R in type position describes a function type -- a $FnType value, the third type flavour alongside $RecType/$TupleType. It is a sibling, not a subtype (a function is not a tuple), carrying the argument types ($params, a reverse-stored list) and the result type ($result).

This is the keystone for describing protocols in fink: a protocol is a record of named function types.

Behaviour

• Declare: Foo = type: fn A, B: R. A sole fn body makes Foo the named function type directly (not a tuple wrapping it).
• Compare: by identity via the existing deep_eq ref.eq fallback -- distinct declarations differ, aliases are equal. No new equality code.
• Guard: a function type used as a match/bind guard is well-defined and runs, but matches nothing yet (instances are unconstructable, so a plain closure is not an instance and falls through).

How

• types.wat: $FnType sibling struct; new_fn_type / fn_type_param / fn_type_result runtime funcs, accreted like the tuple family.
• transform.rs: lower_type_expr mints a $FnType for a fn in type position; a sole-fn type body becomes the named fn type; three new BuiltIns thread the construction.
• scopes/mod.rs: in type position a fn's params are type references, not binding sites -- resolved in the type scope rather than registered as param binds. (This is the one upstream-pass change; it fixes param type-names resolving to unbound.)
• lower.rs / runtime_contract.rs / ir.rs / fmt.rs: emit + contract + CPS-pretty-print wiring for the new builtins.

Deferred (flagged in tests + comments)

• Rendering a bare type value: repr_val / fmt_val have no $Type arm -- only values and instances render today, never type values themselves. Two repr tests pin the target output (Foo fn u8: u8) and are skip'd.
• Typed-function instances: no type_apply $FnType arm. Constructing foo = Foo fn ...: ... (a closure tagged with its type, enabling runtime arg-checking on invocation) is a separate, larger increment touching $Closure and apply.

Tests

Full suite green (1366 lib, 0 failures), clippy clean, zero snapshot drift (no existing test declares a function type, so the new path adds no churn). New fn-type tests: equality, aliasing, guard-falls-through.

[github-actions]

📦 This PR will release v0.89.0 (minor) when merged.