commit 560290ca1d18d4d5a0d204bb187d1aa68c3b5ac7
perf: cache the compiled core module so fresh workers deserialize, not recompile

Workers recycle every 8 requests (calls_until_termination=8). Each fresh worker
ran wasmtime::Module::compile() on the 6.8MB bin/wasm/core.wasm — a ~1.3s
Cranelift JIT — on its first request, so every ~8th request spiked to ~1.3s and
dominated suite wall-clock.

Cache the compiled artifact: on worker core-module load, if bin/wasm/core.cwasm
exists and is newer than core.wasm, load it via Module::deserialize_file() (mmap,
~ms); otherwise Module::compile() as before and atomically (temp+rename) write
the serialized artifact for the next worker. Deserialize failure / stale cache
falls back to a normal compile, so it is self-healing; rebuilding core.wasm
(newer mtime) invalidates the cache. Engine config (epoch_interruption,
signals_based_traps(false)) is unchanged, which the serialized format requires.


commit 83ab9e10f7c0f17805e40a9a3d2b4a3357e5280f
perf: extend the compiled-module disk cache to per-unit modules

Follow-up to 560290c. Per-unit .wasm modules were still Cranelift-compiled per
worker on first use (~40-70ms each), so with 8-call worker recycling fresh
workers re-JIT every unit they touch.

Refactor the core cache logic into a shared WasmWorker helper:
- cached_wasm_path(p): maps <...>.wasm -> <...>.cwasm.
- load_or_compile_cached_module(engine, cached, wasm, bytes, err): deserialize_file
  the .cwasm when it is newer than the .wasm; otherwise Module::compile + serialize,
  written atomically (temp+rename); deserialize failure falls back to compile.
Both the core module load and unit_module() now go through this helper, so unit
artifacts get the same <unit>.uce.cwasm cache the core got.


commit 4f84ac544d9c91a66d068d372055eec8e1723def
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.


