chore: throwaway mode-migration tool for the Implicit cutover (#55, 0121 task 3)

Adds `ail migrate-modes <file>` (a throwaway CLI subcommand) and the AST
walker `ailang_core::ast::for_each_fn_type_mut` it drives: parse a .ail,
map every bare/Implicit fn-type slot to Own, preserve explicit
Own/Borrow, print back. Semantically invisible today (PartialEq treats
Implicit == Own), but it materialises the modes so the post-cutover
parser — which will reject bare slots — accepts the migrated corpus.

Both are throwaway: removed in task 4 once ParamMode::Implicit is
deleted (the closure references Implicit and would not compile).
RED-first: migrate_modes.rs failed to compile (missing walker) before
for_each_fn_type_mut was added. Additive; full workspace suite green.

refs #55

crates/ailang-core/src/ast.rs

@@ -846,6 +846,34 @@ impl Type {
     }
 }
 
+/// Visit every `Type::Fn` in `m`, letting `f` rewrite its modes.
+/// `f(params_len, param_modes, ret_mode)`. Used by the throwaway
+/// `migrate-modes` tool (spec 0062); has no other caller and is
+/// removed if the migration machinery is retired.
+pub fn for_each_fn_type_mut(
+    m: &mut Module,
+    f: &mut impl FnMut(usize, &mut Vec<ParamMode>, &mut ParamMode),
+) {
+    fn walk_ty(t: &mut Type, f: &mut impl FnMut(usize, &mut Vec<ParamMode>, &mut ParamMode)) {
+        match t {
+            Type::Fn { params, param_modes, ret, ret_mode, .. } => {
+                let n = params.len();
+                for p in params.iter_mut() { walk_ty(p, f); }
+                walk_ty(ret, f);
+                f(n, param_modes, ret_mode);
+            }
+            Type::Con { args, .. } => { for a in args.iter_mut() { walk_ty(a, f); } }
+            Type::Forall { body, .. } => walk_ty(body, f),
+            Type::Var { .. } => {}
+        }
+    }
+    for def in m.defs.iter_mut() {
+        if let Def::Fn(fd) = def {
+            walk_ty(&mut fd.ty, f);
+        }
+    }
+}
+
 /// Per-parameter / return mode marker on a [`Type::Fn`]. Full
 /// contract lives in `design/contracts/0008-memory-model.md`.
 ///