refactor: make runtime ABI IR explicit

[baszalmstra]

Why

Reflection metadata was generated through a generic Rust-to-LLVM derive layer. That hid the ABI layout, made target-specific enum encoding hard to audit, and coupled codegen to a proc-macro crate unrelated to Mun semantics.

What

• centralize runtime layouts and constants in AbiTypes and AbiBuilder
• use TypeIdValue and TypeIdGlobal newtypes where LLVM values are easy to mix up
• encode enum payloads with the target pointer width and byte order
• remove the generic value wrappers and mun_codegen_macros
• check size, alignment, and the loaded reflection result

Testing

• pixi run check with RUSTFLAGS=-Dwarnings
• pixi run cargo test -p mun_codegen abi_struct_sizes -- --nocapture
• pixi run cargo test -p mun_codegen --test abi -- --nocapture

[codecov]

Codecov Report

❌ Patch coverage is 94.76031% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.29%. Comparing base (19111bc) to head (3d0a396).

Files with missing lines	Patch %	Lines
crates/mun_codegen/src/ir/types.rs	91.46%	45 Missing ⚠️
crates/mun_codegen/src/ir/types/test.rs	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
+ Coverage   86.74%   87.29%   +0.55%     
==========================================
  Files         263      253      -10     
  Lines       27007    26867     -140     
==========================================
+ Hits        23427    23454      +27     
+ Misses       3580     3413     -167     

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