Replace scons with direct clang++ build

[mpurnell1]

Summary

• Remove scons dependency and replace with build.py that compiles C++/Cython extensions using direct clang++/cythonize subprocess calls
• Supports parallel compilation via subprocess.Popen and incremental builds via -MMD dependency tracking
• Build triggers lazily on first import msgq / import msgq.visionipc (no-op if already built)

Build performance

	Clean build	No-op rebuild
scons -j8	34.5s	~1s
build.py	38.6s	0.18s

Clean builds are comparable (~4s gap is the two sequential Cython compile+link steps). No-op rebuilds are faster since it's just stat checks vs scons' dependency database.

What changed

• Deleted: SConstruct, SConscript, site_scons/ (custom Cython SCons tool)
• Added: build.py — standalone build module with build_msgq(), build_visionipc(), build_test_runners()
• Modified: msgq/__init__.py, msgq/visionipc/__init__.py — call build functions before importing Cython extensions
• Modified: test.sh — scons -j8 → python build.py
• Modified: pyproject.toml — removed scons dependency and site_scons exclusion
• Modified: .gitignore — swapped .sconsign.dblite for *.d (clang dependency files)

Follows the same pattern as commaai/opendbc#3194

Test plan

• ./test.sh passes (build + lefthook lint + pytest + C++ test runners)
• Incremental builds work (mtime + .d depfile checking)
• Clean builds produce identical artifacts
• CI passes on Linux and macOS