feature: Added Backspace and Delete support for clearing selected Not…

[cyberflow]

…ation measures in the selected part while preserving harmony symbols and leaving default whole-measure rests.

Summary

Checks

• Swift tests pass locally or in CI.
• Python helper tests pass locally or in CI.
• I considered whether this changes bundled helper packaging or dependencies.
• I considered whether README, docs, license, or third-party notices need updates.
• I tested the affected user flow manually when UI/audio behavior changed.

Notes