Improve calibration and export workflow receipts

[lishehao]

Summary

• Add hardware-free reference calibration receipts for filename/path, frequency coverage, point count, phase availability, correction state, and edge-clamping warnings.
• Add export receipts for MAT/CSV/TXT/PNG artifacts, output directory, source, correction mode, point count, timestamp, and validation boundary.
• Add a compact non-modal workflow event history.
• Keep receipt-state unit tests portable when Python is installed without Tk.

Replacement

This replaces closed stacked PR #7. Its former base branch was deleted after PR #6 merged, so GitHub could not reopen or retarget it. The same head branch is now based on main.

Validation

• MPLCONFIGDIR=/private/tmp/auto-load-off-test-mpl PYTHONPATH=src python -m unittest discover -s tests (66 tests)
• python -m py_compile on receipt and UI event-handler modules
• git diff origin/main...HEAD --check

Honesty Boundary

This is hardware-free workflow/UI work. It does not add live instrument validation or new real hardware support.