Brummel
39b674c1ec
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