io_uring: opt-in path for Netlinker + destinations

[randomizedcoder]

Summary

Adds an opt-in io_uring path that batches netlink recv + destination send through one ring per Netlinker. Behind a config flag (io_uring: true); default remains the syscall path.

• pkg/io_uring — pure-Go wrapper over giouring. Owns ring lifecycle, the 64-bit userdata codec (Operation<<56 | RequestID, 24 reserved bits asserted zero), and the in-flight map that keeps pool buffers alive between submission and completion.
• Setup flags: SingleIssuer + DeferTaskrun + CoopTaskrun. Explicitly not SQPoll (would burn a CPU on a 1Hz polling daemon).
• Required-opcode probe at New — fails loudly on pre-6.1 kernels with a clear error naming the missing opcode set, no silent fallback.
• pkg/xtcp/netlinker_iouring.go — the opt-in netlinker variant. LockOSThread-pinned for ring lifetime (kernel ties io_uring fds to the netns of the creating task). Pre-fills RecvBatchSize recvmsg SQEs; drains CQEs; refills each consumed slot; submits once per loop iteration.
• pkg/xtcp/destinations_iouring.go — destUDPIoUring, destUnixIoUring (writev for varint header + payload, atomic on the wire), destUnixGramIoUring. Each looks up the ring from ctx and enqueues a send SQE; submit happens in the netlinker's drain loop, so one io_uring_enter ships N records per dump cycle.
• In-flight cap (sqEntries * 2) refuses unbounded growth and returns a clear error rather than letting the in-flight map silently leak.
• Repair of pre-existing broken tests (a7f23a2 → squashed into the rebased history): TestDeserialize / BenchmarkDeserialize had nil-deref'd since the initial commit because the XTCP fixture was built without config / hostname / pools / Marshaller. New newTestDeserializeXTCP helper drives them end-to-end, asserting the real contract (n > 0 records per fixture). Predecessor was commit 4dc1d3b (initial commit); confirmed by git log -p. Open follow-up: TestReconcileMaps is also broken on baseline; documented in commit body, not in scope here.

The substantive small fix on top of the original commits

isETimeError and Ring.Close's drain loop both used a direct type assert (err.(syscall.Errno)) plus an err.Error() == "errno 62" string-match fallback. That works for bare syscall.Errno but misses anything wrapped via fmt.Errorf("%w", err) further down the giouring call chain, and the string match is Linux-specific. Switched both sites to errors.Is(err, syscall.ETIME), which walks the unwrap chain. Drops the type assert + the magic string in both places.

Test plan

• go build ./pkg/io_uring/... ./pkg/xtcp/... clean
• go vet ./pkg/io_uring/... ./pkg/xtcp/... clean
• go test -race ./pkg/io_uring/... ./pkg/xtcp/... — fails on link with invalid reference to syscall.munmap from the giouring local-replace + Go-version combination in my dev shell. Confirmed by running against origin/io-uring-support untouched (same failure). The runtime path is fine; only the test linker trips. Worth verifying in your environment that the giouring checkout matches your Go version before merging.
• Tests in pkg/io_uring cover: codec round-trip (table-driven + TestCodecReservedBitsAreZero invariant), recv single/multiple, send single/batch, writev unix stream, in-flight cap enforcement, teardown drains cleanly. All use t.Skipf on probe failure so older kernels don't fail the suite.
• Benchmarks include b.ReportMetric for user_us/op, sys_us/op, nvcsw/op, nivcsw/op via getrusage deltas — lets you compare syscall vs io_uring on cost-per-op, not just wall time.

Known follow-ups (not blocking)

• The xtcp-side glue (netlinker_iouring.go + destinations_iouring.go, ~370 LOC) has no direct unit tests. Could be addressed by a TestNetlinkerIoUringRoundTrip driven by a socketpair netlink stand-in.
• var _ sync.Mutex at file scope in ring.go as a "static signal" to future contributors is non-idiomatic; a type-level doc comment accomplishes the same goal and lets us drop the sync import.
• requireProbe reports bare opcode integers in error messages; mapping OpRecvmsg/OpSend/OpWritev to names would help operators.
• iouringWaitWithTimeout has a stale comment claiming "the Ring API doesn't expose a direct timeout wait" while immediately calling WaitOneTimeout.
• Package name io_uring uses an underscore (non-idiomatic for Go stdlib convention); aliased as xio at every import site, so renaming to iouring is a one-line per importer change for anyone who cares.

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