feat: support cxx source files in source scanner

[zackees]

Context

The source scanner currently recognizes .cpp, .c, .s/.S, and .cc source files. Test-support comments imply .cxx source files are also expected, but the scanner does not appear to include .cxx.

Relevant area:

• crates/fbuild-build/src/source_scanner.rs
• crates/fbuild-test-support/src/mini_framework.rs

Proposal

Add explicit .cxx source file support to the scanner so .cxx files are compiled like other C++ sources.

Acceptance criteria

• .cxx files are discovered by source scanning.
• .cxx files are compiled as C++ sources.
• Tests cover .cxx discovery and compilation path selection.
• Docs/comments are updated so supported source extensions are consistent.

Open questions

• Should .c++ also be supported, or only .cxx?