Add LOD texture sampling and validate access policy values

[ArthurVasseur]

• Add TextureSampleExplicitLod, exposed as Sampler.SampleLevel(uv, lod)
• Validate access policy values for texture2D[...] and storage[...]

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.43%. Comparing base (6be2f9d) to head (1ee9bd4).

Files with missing lines	Patch %	Lines
src/NZSL/SpirV/SpirvAstVisitor.cpp	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   81.37%   76.43%   -4.95%     
==========================================
  Files         150      207      +57     
  Lines       18694    25769    +7075     
  Branches        0     7613    +7613     
==========================================
+ Hits        15213    19697    +4484     
- Misses       3481     5953    +2472     
- Partials        0      119     +119     

Files with missing lines	Coverage Δ
include/NZSL/SpirV/SpirvAstVisitor.hpp	100.00% <ø> (ø)
...Transformations/ConstantPropagationTransformer.cpp	69.86% <ø> (-0.93%)	⬇️
...rc/NZSL/Ast/Transformations/ResolveTransformer.cpp	83.48% <100.00%> (+0.75%)	⬆️
src/NZSL/GlslWriter.cpp	85.18% <100.00%> (-0.36%)	⬇️
src/NZSL/LangWriter.cpp	87.76% <100.00%> (-0.40%)	⬇️
tests/src/Tests/ComputeTests.cpp	100.00% <100.00%> (ø)
tests/src/Tests/ErrorsTests.cpp	100.00% <ø> (ø)
src/NZSL/SpirV/SpirvAstVisitor.cpp	84.03% <95.23%> (-3.87%)	⬇️

... and 148 files with indirect coverage changes

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[SirLynix]

Thanks!