Add a `pack` variant to IR `TypeInfo` — an ordered, interned sequence of
per-position element types (`PackInfo { elements: []const TypeId }`) — with
constructor (`packType`), structural equality + hashing, and a `pack(T0, …)`
printer. A pack is comptime-only: it lowers to flat positional args before
codegen and has no runtime layout, so `sizeOf` and `toLLVMType` bail loudly
rather than inventing a size. 5 unit tests (N=0/1/3, dedup, order/arity
distinctness, distinct-from-tuple, printer).
Also: give TypeTable an arena for the slices its constructors dupe (freed at
deinit), and add the missing `usize`/`isize` arms to `sizeOf` (a latent
non-exhaustive switch) so types.test.zig compiles and runs leak-free.
