fix: index .cc and .cxx sources and classify .h headers - #93

• Add .cc, .cxx, .hh and .hxx to the walker source extensions, so those files reach the parser and the language counts
• Label a .h header cpp when its text uses a C++ construct, else c
• Label a .h header cpp when a .cc, .cpp or .cxx file shares its directory, preferring the same base name
• Resolve header language in the graph extractor and the parser pool, and bump the cache format so stale C labels are rebuilt once

What and why

How it was checked

• go vet ./... and go test ./... -race pass
• Added or updated tests for behavior changes
• Ran the built binary on a real repo (name it):

Docs

• README, docs/USAGE.md or CHANGELOG.md updated if behavior changed
• Any size or speed claim comes from go run ./bench output
• Planned or unmerged features are not described as shipped