Add VFREEBUSY availability sync

[DoView1]

Summary

• Converts VFREEBUSY-only calendar feeds into synthetic busy events when no VEVENTs are present.
• Skips FBTYPE=FREE slots and preserves detailed VEVENT feeds without duplicating them.
• Adds a small fixture covering restricted free/busy availability.

Validation

• Loaded the shipped ical.js.gs and Helpers.gs in a JS runtime.
• Confirmed the fixture produces two opaque Busy events and skips the FREE slot.
• Confirmed a mixed VEVENT + VFREEBUSY sample still returns only the real VEVENT.

Fixes #336