obs(ml-backtesting): trades=N on progress line for intra-run firing visibility

Adds LobSimCuda::read_total_trade_count (cheap n_backtests*4 byte DtoH of
the trade-log head counters) and emits the sum on each PROGRESS_EVERY
eprintln. Lets future smokes (Q2 CBSW validation, P7 sweep) see trade
firing mid-run instead of waiting for write_artifacts at harness end.

Heads count kernel writes (clamped to ring cap), so the sum is a
monotone trade counter across the sweep — drops back only on harness
reset which doesn't happen mid-run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ml-backtesting/src/harness.rs

@@ -301,9 +301,10 @@ impl BacktestHarness {
             if self.event_count >= next_progress_event {
                 let elapsed = started.elapsed().as_secs_f32();
                 let rate = self.event_count as f32 / elapsed.max(1e-3);
+                let trades = self.sim.read_total_trade_count().unwrap_or(0);
                 eprintln!(
-                    "progress: events={} decisions={} elapsed={:.0}s rate={:.0}ev/s",
-                    self.event_count, total_decisions, elapsed, rate,
+                    "progress: events={} decisions={} trades={} elapsed={:.0}s rate={:.0}ev/s",
+                    self.event_count, total_decisions, trades, elapsed, rate,
                 );
                 next_progress_event += PROGRESS_EVERY;
             }

crates/ml-backtesting/src/sim/mod.rs

@@ -390,6 +390,17 @@ impl LobSimCuda {
         self.n_backtests
     }
 
+    /// Sum of per-backtest trade-log head counters. Cheap DtoH copy of
+    /// `n_backtests * 4` bytes — used by the harness progress line to
+    /// surface intra-run trade firing without reading the full ring.
+    /// Heads count writes (clamped to ring cap inside the kernel), so the
+    /// sum is a reliable monotone trade counter across the sweep.
+    pub fn read_total_trade_count(&self) -> Result<u64> {
+        let mut heads = vec![0u32; self.n_backtests];
+        self.stream.memcpy_dtoh(&self.trade_log_head_d, heads.as_mut_slice())?;
+        Ok(heads.iter().map(|&h| h as u64).sum())
+    }
+
     /// Apply one MBP-10 snapshot's top-10 levels to every backtest's book.
     /// v1 inputs are single-snapshot broadcast (same market data to all
     /// parallel backtests — see spec §7 inference scope, policy sweeps only).