fibers: event-loop Io — real fd readiness via kqueue (B1.4c)

A fiber can block on a file descriptor and the run loop blocks on
kevent until the kernel reports it ready. Reuses the existing
std/net/kqueue.sx bindings. Scheduler gains a lazy kq fd + an
io_waiters list; block_on_fd arms a one-shot EVFILT_READ registration,
records an IoWaiter, and suspends. Run-loop Mode 2: when the ready
queue drains and no timer is pending, block on kq_wait(-1), match each
fired ident to its waiter, evict it, wake the fiber. wake evicts a
pending fd-waiter (cancel_io_waiter_for) so no stale IoWaiter outlives
a reaped fiber.

Adversarial review found two CRITICALs: (1) two fibers on the same fd
share one kqueue registration (macOS EV_ADD replaces), so one is lost
and the loop hangs -- fixed by enforcing one-waiter-per-fd with a loud
abort; (2) an fd-waiter on a never-ready fd 'hangs' -- reclassified as
correct event-loop semantics (a server idling on a socket), with the
misleading orphan-check comment corrected. UAF parity, ident width,
EINTR handling, timer/io precedence all probed safe.

Example: 1816 (pipe roundtrip -- reader blocks, writer writes, reader
wakes via kqueue). macOS only; linux epoll twin deferred. Suite green 754/0.

examples/concurrency/expected/1816-concurrency-fiber-io-pipe.build

@@ -0,0 +1 @@
+{ "target": "macos" }

examples/concurrency/expected/1816-concurrency-fiber-io-pipe.exit

@@ -0,0 +1 @@
+0

examples/concurrency/expected/1816-concurrency-fiber-io-pipe.stderr

@@ -0,0 +1 @@
+

examples/concurrency/expected/1816-concurrency-fiber-io-pipe.stdout

@@ -0,0 +1,2 @@
+log: wrote read 3 [97 98 99]
+n_suspended: 0