Feat/async thread pool

[sinui0]

Closes #371

Replaces the per-task thread-spawning model in mpz-common with a single shared work-stealing async executor. Each task is an async future that owns a deterministic I/O channel; idle workers steal across each other so a long-running task can't pin work on its creator's thread.

• New Executor (built on crossbeam-deque injector + per-worker queues + async_task).
  ExecutorBuilder configures num_threads, prefix, and a custom worker spawn callback (std::thread::spawn by default; web_spawn::spawn on wasm).
• try_join / try_join3 / try_join4 use futures::future::try_join* directly. Closure shape for join / try_join* / map is now for<'a> FnOnce(&'a mut Context) -> BoxFuture<'a, R>.
• Context::map no longer takes a weight closure (load balancing is gone with the new executor).
• Removed: Multithread/MultithreadBuilder/Spawn plumbing, mt/ and st.rs modules,
  load_balance.rs, and Counter.

[sinui0]

@th4s here is what I had in mind.

We should integration test this with tlsn and make sure there are no performance regressions. I would appreciate if you took over this PR

[th4s]

Yeah I will take over.

[th4s]

very nice 👍

I get some decent regression in the browser when testing with the tlsn harness: Seems to be a flat 0.3-0.4 seconds extra offset.

TLSNotary Benchmark Harness: Native (main: 040c6881...)
Running benchmarks from: bench.toml
Output will be written to: metrics.csv

[00:01:07] ████████████████████████████████████████ 18/18 Benchmarks complete
================================================================================
TLSNotary Benchmark Results
================================================================================

cable_mpc [mpc] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  14.46s

cable_proxy [proxy] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  1.59s

fiber_mpc [mpc] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  3.62s

fiber_proxy [proxy] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  0.93s
--------------------------------------------------------------------------------

TLSNotary Benchmark Harness: Native (feat/async-threadpool-integration: 6b0f0338...)
Running benchmarks from: bench.toml
Output will be written to: metrics.csv

[00:01:07] ████████████████████████████████████████ 18/18 Benchmarks complete
================================================================================
TLSNotary Benchmark Results
================================================================================

cable_mpc [mpc] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  14.50s

cable_proxy [proxy] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  1.63s

fiber_mpc [mpc] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  3.61s

fiber_proxy [proxy] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  0.96s
--------------------------------------------------------------------------------

TLSNotary Benchmark Harness: Browser (main: 040c6881...)
Running benchmarks from: bench.toml
Output will be written to: metrics.csv

[00:01:23] ████████████████████████████████████████ 18/18 Benchmarks complete
================================================================================
TLSNotary Benchmark Results
================================================================================

cable_mpc [mpc] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  15.61s

cable_proxy [proxy] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  2.06s

fiber_mpc [mpc] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  4.82s

fiber_proxy [proxy] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  1.35s
--------------------------------------------------------------------------------

TLSNotary Benchmark Harness: Browser (feat/async-threadpool-integration: 6b0f0338...)
Running benchmarks from: bench.toml
Output will be written to: metrics.csv

[00:01:26] ████████████████████████████████████████ 18/18 Benchmarks complete
================================================================================
TLSNotary Benchmark Results
================================================================================

cable_mpc [mpc] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  16.09s

cable_proxy [proxy] (20 Mbps, 20ms latency, 1KB↑ 2KB↓):
  Median:  2.40s

fiber_mpc [mpc] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  5.14s

fiber_proxy [proxy] (100 Mbps, 15ms latency, 1KB↑ 2KB↓):
  Median:  1.68s