fix(typecheck): check enum variant constructor arguments against their payload

[omdxp]

Summary

• ret .Circle("x"), ret Shape.Circle("x"), ret .Rect(1) (too few) and ret Shape.Circle(1, 2) (too many) were all accepted by typecheck. The checker only validated a shorthand's variant name and any nested shorthand, never the constructor's argument count or types.
• A shared check (check_variant_args) now holds a constructor's count and each argument's type to the declared payload, for both the shorthand and the qualified form. A generic enum's parameters, defaults included, are replaced by what the expected type instantiated them as: .Err(1) where a Res<num> is expected reports argument 1 of variant 'Err' of 'Res' expects str, found num. A parameter the expected type does not resolve stands for anything, and a nested shorthand is held to its slot's own enum.
• A standalone qualified constructor (Shape s = Shape.Circle("x")) is checked with no expected type.
• It builds on the per-variant payload types the checker already recorded for nested shorthand checking. The error shows in the editor through the existing diagnostics path.

Test plan

• 3-stage self-compile bootstrap clean: the compiler and stdlib, which build enum values everywhere, type-check under the new rule with no false positives
• fun test . 33/33 files, fun -fmt-check-all clean, fun lint src clean
• scripts/run_examples.sh 218/218 (known pre-existing let_and_lowlevel_types.fn flake)
• New typecheck tests for a wrong type, too few, too many and no arguments (shorthand and qualified), default-filled generic expectations, a qualified constructor against its expected enum, a standalone qualified constructor, a nested shorthand, and an accepting case covering every valid form; plus an fls diagnostic test