feat: request_perf() worker-side timing hostcall; restore demo System Info

Units run in the wasm sandbox, so my_pid/parent_pid/context.server->request_count
read as sandbox stubs — the demo System Info counters were broken, and there was
no authoritative server-side request timing available to unit code (client-side
measurement cannot see queue/dispatch latency).

Add a request_perf() unit API backed by a new uce_host_request_perf hostcall.
The native worker answers it live, returning a DValue:
  worker_pid, parent_pid, request_count,
  accept_us  = (time_start - time_init)*1e6   (entry -> dispatch wait),
  running_us = (now - time_start)*1e6         (since dispatch, live),
  total_us   = (now - time_init)*1e6          (since the request entered UCE),
  workspace_birth_us.
time_init is captured at request entry (handle_request, with a handle_complete
fallback); a RequestPerfSnapshot {pids, request_count, time_init, time_start} is
threaded from wasm_backend_serve through wasm_worker_serve onto the workspace,
and the hostcall computes the live deltas at call time. Wired like uce_host_units
(sized DValue hostcall): core_hostcalls.syms + sys.cpp/sys.h request_perf().

site/demo/index.uce System Info now uses request_perf() and shows the real worker
PID, an incrementing per-worker request count, and the timing counters.

Implemented via the pi agent (gpt-5.3-codex-spark); a review of the live numbers
caught accept_us mistakenly computed as (now - time_init) (== total_us), fixed to
the dispatch wait (time_start - time_init). Independently verified on the host:
System Info shows non-zero PIDs, incrementing count, accept_us ~50us << total_us
~2.4ms with accept+running==total; run_cli_tests --include-wasm-kill => 87 passed,
0 failed, 0 skipped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/wasm/backend.cpp

@@ -147,7 +147,7 @@ String wasm_backend_serve(Request& request, const String& entry_unit, const Stri
 		ctx["ws"]["connection_state"] = request.connection;
 	}
 
-	WasmResponse response = wasm_worker_serve(*g_wasm_worker, ctx, entry_unit, handler);
+	WasmResponse response = wasm_worker_serve(*g_wasm_worker, ctx, entry_unit, handler, &request);
 	if(!response.ok)
 		return(response.error == "" ? String("wasm workspace failed") : response.error);