feat: add buffer recycling to checksum pipeline

[oferchen]

Summary

• Add a recycle channel (Sender<Vec<u8>>) from the main thread back to the I/O thread so consumed buffers are reused instead of allocating fresh ones on every block read
• In pipelined mode, the I/O thread calls try_recv on the recycle channel before each read - if a buffer is available it is reused, otherwise a fresh buffer is allocated as fallback to avoid stalling
• In synchronous mode, a single pre-allocated buffer is reclaimed from current_block on each call, cycling through idle/in-use/reclaimed without allocation after initial setup
• Add comprehensive tests covering various file sizes (empty, sub-block, exact boundary, partial last block, 100 blocks) and property tests verifying pipelined and sequential checksums always match for random data

Closes #1759

Test plan

• CI passes on all platforms (Linux, macOS, Windows)
• pipelined_matches_sequential_various_sizes covers 9 size variants including edge cases
• many_blocks_pipelined_recycling exercises 100 blocks of buffer recycling
• sync_mode_reuses_buffer verifies synchronous buffer reuse
• Property test pipelined_equals_sequential verifies equivalence for random data up to 512 KiB
• Property test reader_collects_all_data verifies no data loss for random data in both modes
• Existing comprehensive tests and benchmarks pass unchanged (API is backward-compatible)