perf(ldk): replace signal poll loop with synchronous sigwait

[erickcestari]

Replaced asynchronous signal handlers (libc::signal + AtomicBool) and a 50ms polling sleep with pthread_sigmask and blocking libc::sigwait.

before:

  aggregate over 5 runs:
    execs/sec: mean 8.8  stddev 0.1  min 8.7  max 8.9
    boot + snapshot: mean 4.041 s
    latency (mean across runs):
      min 107.27 ms  mean 113.35 ms  median 113.30 ms  p99 118.16 ms  max 137.89 ms
    input execution (mean across runs): mean 109.22 ms  median 109.09 ms
    nyx overhead (mean across runs):     mean 4.13 ms  median 4.22 ms
    failed iterations: 0 / 500
    coverage determinism: 83.5% stable (15222 / 92312 edges fluctuated, summed over runs)

INFO  [smite::runners] Reading input from "/input.bin"
DEBUG [smite_scenarios::scenarios::ir] [5.071µs] Executing IR program (40 instructions, 406 input bytes)
DEBUG [smite_scenarios::executor] [119.157µs] SendOpenChannel: 327 bytes
DEBUG [smite_scenarios::executor] [126.085µs] RecvAcceptChannel: waiting
DEBUG [smite_scenarios::executor] [1.911781ms] RecvAcceptChannel: received
DEBUG [smite_scenarios::executor] [7.146892ms] SendFundingCreated: 132 bytes
DEBUG [smite_scenarios::executor] [7.182271ms] RecvFundingSigned: waiting
DEBUG [smite_scenarios::executor] [9.630999ms] RecvFundingSigned: received
DEBUG [smite_scenarios::executor] [9.977985ms] BroadcastTransaction: txid=de0cf6c15ea6cce7a6bc8ad5fe9e83c7545f50586a9f2586817838423ad82bbe
DEBUG [smite_scenarios::executor] [22.79079ms] MineBlocks: mined 6 block(s)
DEBUG [smite_scenarios::executor] [22.882372ms] SendChannelReady: 67 bytes
DEBUG [smite_scenarios::executor] [24.780335ms] RecvChannelReady: waiting
DEBUG [smite_scenarios::executor] [61.01623ms] RecvChannelReady: received
DEBUG [smite_scenarios::scenarios::ir] [61.031084ms] Program executed successfully
DEBUG [smite_scenarios::scenarios::ir] [101.260578ms] Target responded with pong

after:

  aggregate over 5 runs:
    execs/sec: mean 14.2  stddev 0.1  min 14.1  max 14.3
    boot + snapshot: mean 4.029 s
    latency (mean across runs):
      min 67.81 ms  mean 70.41 ms  median 70.14 ms  p99 81.12 ms  max 81.61 ms
    input execution (mean across runs): mean 68.34 ms  median 68.15 ms
    nyx overhead (mean across runs):     mean 2.07 ms  median 1.83 ms
    failed iterations: 0 / 500
    coverage determinism: 80.3% stable (18070 / 91809 edges fluctuated, summed over runs)

INFO  [smite::runners] Reading input from "/input.bin"
DEBUG [smite_scenarios::scenarios::ir] [4.611µs] Executing IR program (40 instructions, 406 input bytes)
DEBUG [smite_scenarios::executor] [113.28µs] SendOpenChannel: 327 bytes
DEBUG [smite_scenarios::executor] [119.826µs] RecvAcceptChannel: waiting
DEBUG [smite_scenarios::executor] [883.175µs] RecvAcceptChannel: received
DEBUG [smite_scenarios::executor] [6.123234ms] SendFundingCreated: 132 bytes
DEBUG [smite_scenarios::executor] [6.157739ms] RecvFundingSigned: waiting
DEBUG [smite_scenarios::executor] [8.2297ms] RecvFundingSigned: received
DEBUG [smite_scenarios::executor] [8.592972ms] BroadcastTransaction: txid=7c52544c5646978c4757c5342bd8bbdbf22c302a932dbd3b7d44602b15d80914
DEBUG [smite_scenarios::executor] [21.934378ms] MineBlocks: mined 6 block(s)
DEBUG [smite_scenarios::executor] [22.055101ms] SendChannelReady: 67 bytes
DEBUG [smite_scenarios::executor] [25.280867ms] RecvChannelReady: waiting
DEBUG [smite_scenarios::executor] [35.929502ms] RecvChannelReady: received
DEBUG [smite_scenarios::scenarios::ir] [35.948273ms] Program executed successfully
DEBUG [smite_scenarios::scenarios::ir] [76.452444ms] Target responded with pong