refactor: track typed codegen values

[baszalmstra]

Why

BasicValueEnum and PointerValue lose the Mun type that determines how a value may be loaded, stored, or projected. Keeping that information beside the LLVM handle makes invalid combinations visible before the opaque-pointer migration removes pointee types from LLVM pointers.

What

• add Operand, Place, and PlaceValue newtypes with narrow responsibilities
• retain Mun and LLVM pointee types across local allocation, load, store, and assignment
• assert type agreement in debug builds without changing generated IR

Testing

• pixi run check with RUSTFLAGS=-Dwarnings
• pixi run cargo test -p mun_codegen --lib

[codecov]

Codecov Report

❌ Patch coverage is 98.95833% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.32%. Comparing base (3d0a396) to head (bd9624a).

Files with missing lines	Patch %	Lines
crates/mun_codegen/src/ir/body.rs	97.72%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           bas/explicit-abi-ir     #605      +/-   ##
=======================================================
+ Coverage                86.42%   87.32%   +0.90%     
=======================================================
  Files                      253      254       +1     
  Lines                    26837    26933      +96     
=======================================================
+ Hits                     23193    23520     +327     
+ Misses                    3644     3413     -231     

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