Preserve explicit struct constructor ABI

[DavidObando]

Summary

• admit explicit init overloads on plain G# structs and emit real CLR value-type constructors
• keep translated C# plain-struct constructors, source parameter names, fields, visibility, and properties instead of primary-constructor/literal reshaping
• preserve same-language calls, overloads, delegation, expression/block bodies, and generic struct calls
• leave classes on Preserve explicit class constructor ABI in cs2gs #2760's path and records/data structs on Fix C# record ABI for data types #2767's separate path

Validation

• targeted value-constructor/ABI/negative-control tests: 35 passed
• Core.Tests: 6,694 passed, 1 skipped
• Compiler.Tests: 3,431 passed
• Cs2Gs.Tests: 1,731 passed (1,730 full batch + isolated pre-existing shared-asset collision test)
• Oahu/ABI targeted suites: 56 passed

Oahu proof

Oahu 200e3b4c26c781368f74bbef098caea89bac8754 contains only two plain structs (LinuxUcred, MacXucred), both field-only with no explicit constructor. Constructor-bearing Oahu value types are records and remain covered by #2767. The Oahu translation/ABI suites and the real C# named-argument/getter/field consumer all pass; no #2766-scoped Oahu ABI failure remains.

Fixes #2766