refactor: make heap projections explicit

[baszalmstra]

Why

Heap references and arrays recovered their pointee types by inspecting LLVM pointer element types. Opaque pointers remove that information, and unchecked wrapper construction let the wrong pointer depth travel until a later load or GEP failed.

What

• store the object or array aggregate type beside each runtime handle
• return typed PlaceValues from heap, field, and array projections
• remove unchecked constructors and all get_element_type calls from codegen
• route field and element loads and stores through typed places

Testing

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

[codecov]

Codecov Report

❌ Patch coverage is 97.40260% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.14%. Comparing base (bd9624a) to head (c65a8f3).

Files with missing lines	Patch %	Lines
crates/mun_codegen/src/ir/body.rs	97.84%	2 Missing ⚠️
crates/mun_codegen/src/ir/array.rs	96.66%	1 Missing ⚠️
crates/mun_codegen/src/ir/reference.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                      @@
##           bas/typed-codegen-values     #606      +/-   ##
============================================================
- Coverage                     87.32%   87.14%   -0.19%     
============================================================
  Files                           254      254              
  Lines                         26933    26887      -46     
============================================================
- Hits                          23520    23430      -90     
- Misses                         3413     3457      +44     

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