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
