refactor(driver): record multishot results in ops

[Berrysoft]

Closes #746

[Copilot]

Pull request overview

This PR refactors io-uring multishot handling so that per-shot results (and selected managed buffers) are recorded inside the operation itself, addressing the buffer-loss scenario described in #746 when a managed op completes after cancellation.

Changes:

• Move multishot result storage from the io-uring driver (HashMap<ErasedKey, VecDeque<_>>) into the op implementations via new OpCode::{push_multishot,pop_multishot} hooks.
• Extend io-uring OpCode::set_result to receive &io::Result<usize> and &Extra, allowing ops (notably managed-buffer ops) to record/own selected buffers for later cleanup.
• Refactor io-uring buffer pool internals to be Clone/Rc-backed and introduce an owned buffer handle that returns buffers on drop.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
compio-driver/src/sys/stub/mod.rs	Removes no-longer-needed cleanup_multishot stub API.
compio-driver/src/sys/poll/mod.rs	Removes no-longer-needed cleanup_multishot for polling driver.
compio-driver/src/sys/iour/op.rs	Implements per-op multishot queues and records selected buffers in managed ops.
compio-driver/src/sys/iour/mod.rs	Routes multishot CQEs directly into ops; removes driver-level multishot storage; adjusts buffer-pool release.
compio-driver/src/sys/iocp/mod.rs	Removes no-longer-needed cleanup_multishot for IOCP driver.
compio-driver/src/sys/fusion/op.rs	For fused ops, forwards the expanded io-uring OpCode surface (fallback/blocking/result/multishot).
compio-driver/src/sys/fusion/mod.rs	Removes driver-level cleanup_multishot forwarding.
compio-driver/src/op.rs	Renames managed fallback buffer type to FallbackOwnedBuffer to disambiguate with io-uring owned buffers.
compio-driver/src/lib.rs	Removes cleanup_multishot calls during pop/pop_with_extra since storage moved into ops.
compio-driver/src/key.rs	Calls OpCode::set_result(&Result, &Extra) for io-uring completions to let ops capture selected resources.
compio-driver/src/buffer_pool/iour.rs	Makes io-uring buffer pool Clone (Rc-backed) and introduces an owned buffer handle with drop-based reuse.
compio-driver/src/buffer_pool/fusion.rs	Renames/re-exports owned buffer types (FallbackOwnedBuffer, IoUringOwnedBuffer) for fusion builds.
compio-driver/src/buffer_pool/fallback.rs	Exposes FallbackOwnedBuffer re-export for non-fusion builds.

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[George-Miao]

With BufferPools internalized into Ops now, does that mean maybe we can refactored Managed api and return buffers directly from returned Ops? i.e., TakeBuffer won't need BufferPool arg etc.

[George-Miao]

LGTM