feat: std.thread — Thread, Mutex/Cond, bounded worker Pool (PLAN-HTTPZ S6)

pthread bindings with darwin opaque sizes (mutex 64B, cond 48B; glibc
divergence is a C3 per-OS item). Mutex/Cond initialize IN PLACE and
Pool lives behind Pool.create's heap pointer — POSIX sync objects are
address-sensitive, so nothing here moves after setup. Thread.spawn
takes the C2 re-entry contract entry (callconv(.c), fabricates its own
Context); Pool workers do exactly that with a per-worker malloc-backed
GPA, then run default-conv tasks inside it. submit returns false on a
full backlog (httpz thread_pool backpressure); shutdown finishes
queued work and joins every worker.

examples/1637 pins: 4 raw threads x 1000 locked increments, 100 pool
tasks summing exactly once across 4 workers, a held worker + full
backlog refusing the next submit, clean shutdown. JIT + AOT (AOT run
three times). The std.sx barrel carries thread; .ir snapshot regen is
the usual renumbering.

examples/expected/1637-std-thread.stdout

@@ -0,0 +1,4 @@
+raw threads: 4 x 1000 locked increments = 4000
+pool: 100 tasks across 4 workers summed to 5050
+backpressure: full backlog refuses
+std.thread ok