fix: treat zero-duration clips as empty instead of one-frame spans

[ubugeeei]

Finding

Zero-duration clips are normalized into a one-frame span. That can shift timeline duration, audio planning, and hidden probe behavior.

Evidence

• src/lib/clip.tsx clamps clip end behavior.
• src/lib/vue.tsx has matching timeline logic.

Acceptance criteria

• Represent zero-duration clips as empty.
• Keep existing positive-duration behavior unchanged.
• Update audio/timeline calculations consistently.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。