## Why
Codex syntax-highlights diffs for conventional C++ extensions such as
`.cpp` and `.cxx`, but C++ module interface files using `.cppm`, `.ixx`,
or `.cxxm` fall back to plain diff coloring. The bundled syntax set
already includes C++, but it does not resolve those module extensions by
itself.
Closes#28223.
## What changed
- map `.cppm`, `.ixx`, and `.cxxm` to the existing `cpp` syntax in
`render/highlight.rs`
- extend alias-resolution coverage for all three module extensions
- verify `.cpp`, `.cppm`, `.ixx`, and `.cxxm` diffs produce
syntax-highlighted RGB spans while unknown extensions retain the plain
fallback
- snapshot the syntax-colored token segmentation for the supported C++
module extensions
## How to Test
1. Ask Codex to create or modify a C++ module interface file using
`.cppm`, `.ixx`, or `.cxxm`.
2. Confirm C++ tokens in the rendered diff receive syntax colors instead
of only the red/green diff treatment.
3. Modify an equivalent `.cpp` file and confirm its existing
highlighting remains unchanged.
4. Modify a file with an unknown extension and confirm it still uses the
plain diff fallback.
Targeted tests:
- `just test -p codex-tui -E
'test(find_syntax_resolves_languages_and_aliases) |
test(cpp_module_extensions_use_cpp_highlighting) |
test(unknown_extension_falls_back_without_syntax_highlighting)'`
Felipe Coury
3ded846488