The reader's line_index and file_size were frozen at open time.
After current_line exceeded the initial 150K lines, get_line_impl
returned None for all subsequent reads. With the background thread
appending ~10K lines/sec, ~40% of measured frame latencies were
actually the cost of a None return, not real I/O.
- Add PreadReaderCore::refresh_index(&mut self): seek to start,
rebuild LineIndex, update file_size, invalidate read cache
- Add PreadReaderPlain::refresh_index forwarding method
- Add ReadCache::invalidate to force cache miss after reindex
- Rewrite bench_scroll_during_append: time-based refresh (250ms),
only record latencies for successful reads, assert max_line > initial
- Add regression tests for refresh_index with appended lines
Replace `static mut OLD_SIGBUS_HANDLER` with AtomicU8 + AtomicPtr to
remove data race UB when concurrent benchmarks call open() from multiple
threads.
Key changes:
- Use `Once::call_once` to guarantee single handler installation
- Publish old handler to atomics BEFORE installing new handler (closes
the handler-active-but-state-unpublished race window)
- Read atomics with Acquire in signal handler (async-signal-safe)
- Align si_addr to page boundary before mmap(MAP_FIXED)
- Add concurrent test: 8 threads open all 5 variants simultaneously
