Support backend-defined jobs in reflection hooks

[maleadt]

GPUCompiler's reflection macros currently expect a CompilerJob, tying them to GPUCompiler's compilation pipeline. Backends such as cuTile have their own job types and compilation stages, but still need to inspect kernels launched by an expression—including expressions that use multiple backends.

Allow the individual @device_code_* macros to accept backend-defined jobs and dispatch to their reflection methods. Document compile_hook as the shared entry point, including how backends report cached compilations. Backends can also build their own stage macros with an optional job filter, and display already-inferred code with type-instability highlighting. This lets them preserve constant specializations instead of reconstructing a generic inference request.

Missing LLVM or native reflection produces an explicit unsupported-operation error. The all-stage @device_code directory dump remains specific to GPUCompiler's LLVM pipeline.

Bump the version to 2.6 for downstream compatibility bounds.

[codecov]

Codecov Report

❌ Patch coverage is 61.33333% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.76%. Comparing base (0432e4c) to head (6981f0a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/reflection_compat.jl	53.22%	29 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #925      +/-   ##
==========================================
+ Coverage   85.49%   85.76%   +0.26%     
==========================================
  Files          29       29              
  Lines        5584     5598      +14     
==========================================
+ Hits         4774     4801      +27     
+ Misses        810      797      -13     

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