feat: configurable chapter window & parallel metadata parsing with benchmark
1. Configurable chapter window size (onDemandChapterWindowSize: 3-15)
- Parameterized window radius in RDEPUBChapterRuntimeStore
- Updated RDEPUBChapterWindowCoordinator to use configurable radius
- RDEPUBChapterWindowSnapshot.from() accepts chapter array instead of fixed prev/next
- Even numbers round up to odd (4→5), min 3, max 15
2. Configurable metadata parsing concurrency (metadataParsingConcurrency)
- Default equals CPU core count
- Parallel execution via OperationQueue in paginateMetadataOnly
- Each worker creates independent builder instance
- NSLock protects result aggregation
3. Per-chapter and total wall-clock timing instrumentation
- Separated render vs I/O timing per chapter
- Summary log with wallClockMs, renderTotalMs, writeTotalMs, avgRenderMs
- Timing stored in RDEPUBReaderContext for test access
4. UI automation test infrastructure
- Added --demo-window-size, --demo-concurrency, --demo-clear-cache launch args
- DemoReaderState exposes windowSize, parseMs, parseConcurrency
- ConfigurableWindowTests: 5 test cases for window size 3/5/15
- ConcurrentParsingTests: 4 test cases for concurrency 2/4
- MetadataParseBenchmarkTests: serial vs parallel benchmark
5. Bug fixes
- Fixed page snap-back during background parsing (isUserInteracting check)
- Reduced BookPageMap refresh frequency from 16 to 32 chapters
- Moved waitForReadingInteractionToSettle outside operation loop
6. Design doc: dual-layer PageMap (estimated + precise mixed)
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