Replace O(n²) lookups with O(n) preprocessing:
1. createTocEntry(): Build href->spineIndex map once in beginTocPass()
instead of scanning spine file for every TOC entry
2. buildBookBin(): Build spineIndex->tocIndex vector in single pass
instead of scanning TOC file for every spine entry
For 2768-chapter EPUBs, this reduces:
- TOC pass: from ~7.6M file reads to ~5.5K reads
- buildBookBin: from ~7.6M file reads to ~5.5K reads
Memory impact: ~80KB for href map (acceptable trade-off for 10x+ speedup)
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