DOC: Add compiler-cautions.md with pitfalls

Distilled from review of 1,296+ COMP: commits (2019-2026), GitHub PR
review comments, and CDash failure analysis. Covers:

- Smart pointer / forward-declared type hazards (GCC 7-9.1 brace-init
  bug, destructor-in-header UB with unique_ptr<Incomplete>)
- Template deduction pitfalls (missing typename, non-type param type
  mismatch, constexpr static ODR-use in lambdas)
- constexpr edge cases (MSVC x86 if constexpr, function pointer arrays)
- Undefined behavior patterns (bit shift overflow, implicit truncation,
  empty-vector data(), uninitialized loop vars)
- Platform specifics (char signedness on ARM, missing <cstdint> on
  GCC 13+, reserved identifiers on AppleClang)
- GCC warnings (-Wmaybe-uninitialized false positives on ARM64,
  -Wdeprecated-copy Rule of Zero violations)
- MSVC C4805 bool/int mixing, constexpr arrays
- Clang suppression macros for third-party headers
- ITK deprecated API replacements (ITK_LEGACY_REMOVE=ON breaks)
- Shared library symbol visibility (ITK_TEMPLATE_EXPORT)
- Python binding removals (np.bool, PySequence_Fast_GET_ITEM)
- 13-item refactoring checklist

Add routing entry to AGENTS.md table.

Document that refactoring should not introduce new clang-tidy diagnostics,
and enumerate check families that conflict with ITK coding standards
(llvmlibc-*, readability-identifier-length, google-*, etc.).

AI agents must consult compiler-cautions.md, compile the changed
module, and run relevant tests before committing C++ code.
Update AGENTS.md routing row for commit tasks to surface these docs.

Author: hjmjohnson

AGENTS.md

@@ -15,8 +15,9 @@ Load only what your task requires:
 | Writing or running tests | [./Documentation/AI/testing.md](./Documentation/AI/testing.md) |
 | Code style, naming conventions | [./Documentation/AI/style.md](./Documentation/AI/style.md) |
 | Writing ITK C++ classes, CMake build configuration, and Python wrapping configuration | [./Documentation/AI/conventions.md](./Documentation/AI/conventions.md) |
-| Creating a git commit | [./Documentation/AI/git-commits.md](./Documentation/AI/git-commits.md) |
+| Creating a git commit with C++ changes | [./Documentation/AI/git-commits.md](./Documentation/AI/git-commits.md), [./Documentation/AI/compiler-cautions.md](./Documentation/AI/compiler-cautions.md), [./Documentation/AI/building.md](./Documentation/AI/building.md), [./Documentation/AI/testing.md](./Documentation/AI/testing.md) |
 | Opening or updating a pull request | [./Documentation/AI/pull-requests.md](./Documentation/AI/pull-requests.md) |
+| Avoiding compiler-specific pitfalls and refactoring hazards | [./Documentation/AI/compiler-cautions.md](./Documentation/AI/compiler-cautions.md) |
 
 ## Critical Pitfalls