Redesign benchmark to measure sustained (thermally throttled) hashrate

instead of initial burst performance. Previously the benchmark used a
fixed 20s warmup + 10s peak measurement, which reported inflated
results on thermally constrained hardware (e.g. 179 H/s vs actual
sustained 117 H/s on a MacBook Pro).

- Adaptive warmup with stability detection: mine for at least 90s,
  then compare rolling 10s hashrate windows. Require 3 consecutive
  windows within 5% before declaring thermal equilibrium (cap 300s)
- Average-based measurement: record mean hashrate over 30s instead
  of peak, reflecting real sustained throughput
- Start candidates at half the system cores — lower thread counts
  are rarely optimal and waste time warming up
- Add CoolingDown phase: 5s idle pause between tests so each starts
  from a similar thermal baseline
- Adaptive time estimates: use observed warmup durations from
  completed tests to predict remaining time
- UI shows Stabilizing when waiting for thermal equilibrium past
  the minimum warmup, Cooling during idle pauses"

src/util/i18n.cpp

@@ -879,6 +879,8 @@ void I18n::loadBuiltinEnglish()
         strings_["mining_reset_defaults"] = "Reset Defaults";
         strings_["mining_benchmark_tooltip"] = "Find optimal thread count for this CPU";
         strings_["mining_benchmark_testing"] = "Testing";
+        strings_["mining_benchmark_cooling"] = "Cooling";
+        strings_["mining_benchmark_stabilizing"] = "Stabilizing";
         strings_["mining_benchmark_cancel"] = "Cancel benchmark";
         strings_["mining_benchmark_result"] = "Optimal";
         strings_["mining_benchmark_dismiss"] = "Dismiss";