feat: support cxx source scanning

[zackees]

Summary

• add .cxx to sketch, core, and variant source scanning
• keep .cxx routed through existing C++ compile database handling
• update scanner docs/comments and add scanner coverage for .cxx

Closes #651

Tests

• soldr cargo fmt
• soldr cargo test -p fbuild-build source_scanner
• soldr cargo test -p fbuild-build compile_database::tests::generate
• soldr cargo clippy -p fbuild-build --all-targets -- -D warnings
• soldr cargo test -p fbuild-build

Note: an initial combined focused-test command and the first clippy run hit tool timeouts while compiling. The background cargo/rustc processes were allowed to finish, then the same focused tests and clippy command were rerun successfully.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: d511635c-7ce4-4fd5-b1eb-e814b085b4d2

📥 Commits

Reviewing files that changed from the base of the PR and between 8da6457 and 05e1e37.

📒 Files selected for processing (2)

• crates/fbuild-build/src/README.md
• crates/fbuild-build/src/source_scanner.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/651-cxx-source-scanner

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