chore: clean cancelled read requests earlier in driver state transitions

[caicancai]

Handle cancelled reads earlier in on_event (Request/Polled) and keep requests_by_offset in sync when cancellation is detected before poll promotion.

Purpose
When read futures are dropped quickly around registration/poll transition, cancelled requests no longer linger in internal state. This reduces stale index entries, avoids repeated coalescing scans over requests that will never run, and keeps queue/index state more consistent without changing existing dequeue/coalescing correctness checks.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 34.26%

_{Comparing caicancai:fix/read-early-cancel-cleanup (57159f2) with develop (d1097c5)}

Summary

⚡ 1 improved benchmark
✅ 1137 untouched benchmarks
⏩ 1265 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	chunked_bool_into_canonical[(1000, 10)]	88.4 µs	65.9 µs	+34.26%

Footnotes

1. 1265 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩