Three optimizations for EPUBs with many chapters (e.g. 2768 chapters):
1. OPF idref→href lookup: Build sorted hash index during manifest parsing,
use binary search during spine resolution. Reduces ~4min to ~30-60s.
2. TOC href→spineIndex lookup: Build sorted hash index in beginTocPass(),
use binary search in createTocEntry(). Reduces ~4min to ~30-60s.
3. ZIP central-dir cursor: Resume scanning from last position instead of
restarting from beginning. Reduces ~8min to ~1-3min.
All optimizations only activate for large EPUBs (≥400 spine items).
Small books use unchanged code paths.
Memory impact: ~33KB + ~39KB temporary during indexing, freed after.
Expected total: ~17min → ~3-5min for Shadow Slave (2768 chapters).
Also adds phase timing logs for performance measurement.
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