runtime,syscall,internal/poll,os: wasip1 poll_oneoff scheduler integration + net.FileListener (#5386)

* runtime,syscall,internal/poll,os: wasip1 poll_oneoff scheduler integration + net.FileListener

On wasip1 today every syscall.Read/Write blocks the entire wasm module — the
cooperative scheduler invokes poll_oneoff only for sleep/timer wakeups, and
there's no path from the net package to a working TCP server. This change
fixes both: it threads poll_oneoff through the scheduler's idle path so a
goroutine doing FD I/O parks instead of blocking the module, and it provides
enough internal/poll / os / syscall surface that upstream Go's
net.FileListener / net.FileConn works on a host-pre-opened TCP socket.

* runtime: keep scheduler_cooperative idle-wait calls direct

TestBinarySize/hifive1b/examples/echo regressed by 32 bytes after the
previous commit routed the scheduler's idle wait through a
schedulerIdleWait helper. The extra call frame + branch landed on every
non-wasip1 cooperative target, where the original direct sleepTicks /
waitForEvents calls compile to a single inlined call.

src/internal/poll/export_test_wasip1.go

@@ -0,0 +1,45 @@
+//go:build wasip1
+
+// Internal-test helpers exposed via go:linkname so user code can drive
+// the deadline-aware Read/Write loop without becoming a stdlib package.
+// Not part of the public API; the names are intentionally awkward to
+// signal "for tests only".
+
+package poll
+
+import (
+	"syscall"
+	"time"
+)
+
+// pollTestReadWithDeadline opens a pollable FD wrapper for sysfd, sets
+// a read deadline d into the future, calls Read once, and returns
+// (n, err). Caller is responsible for closing sysfd.
+//
+//go:linkname pollTestReadWithDeadline
+func pollTestReadWithDeadline(sysfd int, d time.Duration, p []byte) (int, error) {
+	fd := &FD{Sysfd: sysfd, IsStream: true}
+	// Best-effort init; ignore error so a caller using a not-fcntl-able FD
+	// (stdin under wazero, etc.) still gets to test the deadline path on
+	// whatever park behaviour the runtime gives.
+	_ = fd.Init("test", true)
+	if err := fd.SetReadDeadline(time.Now().Add(d)); err != nil {
+		return 0, err
+	}
+	return fd.Read(p)
+}
+
+// pollTestSetNonblock toggles O_NONBLOCK on a raw sysfd. Useful in
+// tests when the caller wants to ensure the FD is in nonblocking mode
+// before calling pollTestReadWithDeadline (Init is best-effort and may
+// silently skip).
+//
+//go:linkname pollTestSetNonblock
+func pollTestSetNonblock(sysfd int) error {
+	flags, err := syscall.Fcntl(sysfd, syscall.F_GETFL, 0)
+	if err != nil {
+		return err
+	}
+	_, err = syscall.Fcntl(sysfd, syscall.F_SETFL, flags|syscall.O_NONBLOCK)
+	return err
+}