Separate (delayed) responses (roadmap 1.2)

[cvik]

Summary

Handlers can now defer CON responses — the server sends an empty ACK immediately and delivers the actual response later as a separate CON with retransmission.

API

fn slow_handler(req: coap.Request) ?coap.Response {
    const deferred = req.deferResponse() orelse return Response.ok("sync");
    my_worker.submit(deferred, req.payload());
    return null; // server sends empty ACK
}
// Later, from any thread:
deferred.respond(coap.Response.ok(result));

Implementation

• deferred.zig: Pre-allocated pending pool (default 16 slots), lock-free MPSC queue, DeferredResponse handle with respond()/cancel()
• handler.zig: Request.deferResponse() method, DeferContext type
• Server.zig: Config.max_deferred, pool init/deinit, tick-loop queue drain + CON retransmit with exponential backoff, ACK/RST handling for separate responses

Performance

• Zero overhead for synchronous handlers (one null check on deferred field, skipped queue drain when count_active == 0)
• Bench regression check: no measurable change in plain CON throughput

Test plan

• zig build test — all tests pass (8 new deferred pool tests + existing 120+)
• Bench regression: plain CON 1T within normal range (~600-720K req/s)