fix: include modified mounted files in generated output (closes #56) (#62)

Previously, _filter_generated_files unconditionally excluded ALL files
whose names matched uploaded (mounted) files. This meant that if a user
uploaded example.xlsx and the code modified it, the modified file would
not appear in the execution output.

Changes:
- Go runner: add mod_time (Unix timestamp) to FileInfo struct and
  populate it in HandleList and HandleUpload responses
- _detect_generated_files: pass mod_time through to callers
- _filter_generated_files: accept execution_start_unix parameter;
  keep mounted files whose mod_time >= execution start time
- Add comprehensive unit tests for the new filtering behavior
- Add Go test verifying mod_time presence in file listings