[Feature] ABI: Add proper support for const generic structs

[mitchmindtree]

The ABI currently does not properly represent const generic structs. When a const generic struct is used in a transition, it appears in its monomorphized form rather than preserving the generic type with concrete arguments.

Example

struct Foo::[N: u32] { a: [u32; N] }

transition bar(f0: Foo::[42]) -> Foo::[42] { .. }

Desired behavior

• Type declarations should include const parameters (e.g., Foo with parameter N: u32).
• Type references in transitions should include concrete arguments (e.g., Foo::[42]).

Changes needed

• Add const_params: Vec to Struct for type declarations.
• Add const_args: Vec to StructRef for type references.

Possible Approach

May require a multi-phased ABI generation:

1. After type-checking: capture generic type declarations with their const parameters.
2. After ConstPropUnrollAndMorphing: capture concrete const arguments in transition signatures.

Related discussion: #29064