Fix recurrence engine performance (19-907x slower than rrule)

[thedhanawada]

Problem

Benchmark data shows forceCalendar's recurrence engine is 19-907x slower than the rrule library. This is unacceptable for an enterprise calendar.

Key Bottlenecks to Investigate

• findDSTTransitions checks every day in the range (should binary search)
• Occurrence expansion may be recalculating timezone offsets unnecessarily
• Cache hit rates should be measured and optimized
• The 100-entry LRU cache for expanded occurrences may be too small

Benchmark Results (current)

• Daily recurrence: 19x slower
• Weekly with BYDAY: 907x slower
• Monthly: 40x slower

Target

Within 5x of rrule for all patterns, ideally within 2x.