analyze: IOSQE_CQE_SKIP_SUCCESS for fire-and-forget ops (cancels now, close-direct later)

[MDA2AV]

Severity: low / enhancement — analysis first; the win is micro today but grows with #91.

What it is

IOSQE_CQE_SKIP_SUCCESS (SQE flag 1 << 6, kernel 5.17+): a successfully-completing op posts no CQE; failures still post. For fire-and-forget ops it removes one CQE reap + dispatch per op.

Where it fits today

• ASYNC_CANCEL SQEs — the only current candidate. Cancel completions are already ignored (case KindCancel: return; — Reactor.Loop.SharedRing.cs#L177-L178), so skipping the success CQE is pure savings. Note cancels frequently "fail" benignly (-ENOENT target already completed, -EALREADY) — those still post and stay ignored; that's fine.

Where it must never be used

recv (multishot — CQEs are the data path), all sends (completions drive WriteHead, partial-send resubmit, CompleteFlush, ZC notifs), accept, the eventfd wake poll, and OP_TIMEOUT (its normal completion is -ETIME, a failure code, so skip changes nothing and only invites confusion).

Where it gets interesting later

• perf: adopt direct descriptors (registered file table) for accepted sockets #91 close-direct: one fire-and-forget IORING_OP_CLOSE per connection — at high connection churn that's real CQE volume worth skipping (log failures, which still post).
• Any future linked chains where intermediate success CQEs are noise.

Suggested analysis before implementing

1. Add per-kind CQE counters (observability: per-reactor counters (ENOBUFS, overflows, accepts, sheds, pool hit rate) #101) and measure what fraction of reaped CQEs are ignorable — sizes the win with data.
2. Feature-gate: on pre-5.17 kernels the flag yields -EINVAL per op; probe once at startup and fall back to plain SQEs.
3. Verify flag interactions on the target kernel (notably with linked SQEs / drain semantics) before enabling anywhere near a chain.
4. Invariant to preserve: nothing in the runtime may wait on or count a completion from a skipped op. Today cancels have no such accounting — keep it that way, and document the rule.