📝 Adopt LLVM-style C++ documentation comments for v4

[burgholzer]

🤖 AI text below 🤖

Goal

Replace the 1,615 project-owned block Doxygen comments with LLVM-style /// before v4, without behavior or prose churn.

Migration

• include/**
• mlir/include/**
• MLIR implementations and tools, plus bindings/mlir/**
• Remaining implementations, bindings, and tests
• Add a dependency-free gate rejecting /** and explicit \brief or \details commands in project-owned C++ files

Preserve useful prose and other Doxygen commands. Keep each migration PR mechanical and behavior-free.