fix: ignore line comments in parser diagnostics

[swananan]

Summary

• stop the unknown-keyword heuristic at // comment boundaries outside strings
• preserve // inside grammar-valid trace file paths by validating the parsed trace_pattern and file_path span
• preserve the underlying Pest diagnostic for real syntax errors after commented lines
• add regressions for comma-containing comments and repeated slashes in trace paths

Testing

• cargo fmt --all -- --check
• cargo test -p ghostscope-compiler (185 passed)
• cargo clippy --all-targets --all-features -- -D warnings
• standard host-to-host e2e via runner job 8cab17c08bea (exit code 0)

Container-topology e2e was intentionally skipped because this parser-only change does not affect container behavior, PID namespaces, sandbox topology, Docker execution, or runner semantics.