Add Gentle and YouTube caption format support

[Tillingxianyu]

Fixes #160

Summary:

• Added Gentle JSON export/import based on the existing hypertranscript word timings.
• Added YouTube automatic captions XML export/import using SRV3-style word timing nodes, with fallback support for transcript text nodes.
• Added a YouTube AC VTT export and taught the existing VTT import path to preserve inline per-word timestamps.

Testing:

• node --check js/hyperaudio-lite-editor-export.js
• Chrome headless smoke test for Gentle JSON, YouTube XML, and YouTube VTT round trips.

Bounty payout:
PayPal: 1434042923@qq.com
EVM: 0x52d019e01fa43feba25e614a227aae01d600dc71