Replace O(n*m) pandas operations with O(n log m) interval tree queries
for speaker assignment, where n = words/segments and m = diarization segments.

Performance improvement:
- 7-minute video (1185 words, 147 segments): 73.9s -> 0.32s (228x faster)
- 3-hour podcast: Minutes of processing -> Seconds

Changes:
- Add IntervalTree class using sorted array + binary search
- Refactor assign_word_speakers to use interval tree for overlap queries
- Maintain backward compatibility with same function signature
- Identical output to original implementation

The interval tree uses numpy arrays for efficient storage and binary search
(np.searchsorted) for O(log n) candidate finding, then filters candidates
for actual overlaps.

Fixes #1335