feat: std.http pooled handler dispatch (PLAN-HTTPZ S7b)

thread_pool_count = 0 (default) keeps handlers inline on the loop
thread — the measured fast path (BENCH-HTTPZ.md). N > 0 dispatches
each parsed request to a std.thread Pool of N workers, completing the
httpz two-pool shape: the connection freezes as CONN_HANDLING (no
reads, growth, eviction, or recycling — the worker borrows views into
its read buffer), the worker runs the handler under a per-job arena
and serializes into job-owned bytes, the completion queues under the
PoolState mutex, and the loop wakes through the new std.event wake
channel (kqueue EVFILT_USER + EV_CLEAR; the epoll twin maps to
eventfd), attaches the response, compacts the buffer, and resumes
keep-alive/pipeline handling. A full backlog sheds with 503. Stale
completions (generation mismatch after close) are dropped. Pool mode
requires the server's constructing allocator to be thread-safe
(GPA/malloc), documented on the knob.

PoolState lives behind a heap pointer (it embeds a Mutex and is shared
with workers; the Server struct itself is returned by value).
serialize_response/run_handler_job share one serialize_bytes.

examples/1633 gains the pooled section (GET, body echo, 404 across
worker threads) plus the loop-wake path exercised end to end; AOT run
five times. examples/1632 unchanged but the Event struct gains `user`.

library/modules/std/net/kqueue.sx

@@ -39,6 +39,10 @@ kevent :: (kq: i32, changelist: *Kevent, nchanges: i32, eventlist: *Kevent, neve
 EVFILT_READ  :i16: -1;
 EVFILT_WRITE :i16: -2;
 EVFILT_TIMER :i16: -7;
+EVFILT_USER  :i16: -10;
+
+// EVFILT_USER fflags
+NOTE_TRIGGER :u32: 0x01000000;
 
 // Action/state flags (darwin)
 EV_ADD     :u16: 0x0001;