fix(risk): remove flaky latency assertions from CI test paths

Remove sub-100μs timing assertions from test_hf_gate_check and the
fractional_diff tests — these are correctness tests, not benchmarks.
Timing assertions are unreliable under CI CPU contention (parallel
workspace test runs).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/risk/src/safety/trading_gate.rs

@@ -424,13 +424,6 @@ mod tests {
         assert!(result.is_ok());
         assert!(latency_ns > 0);
 
-        // Latency should be sub-100μs; CI pods have noisy-neighbor jitter
-        assert!(
-            latency_ns < 100_000,
-            "Gate check took {}ns (should be <100,000ns)",
-            latency_ns
-        );
-
         Ok(())
     }