test(stress): raise default FixtureTimeout 15s -> 30s (#408)

Eliminates the dominant stress-failure mode (CI VM variance overshooting
tight per-fixture budgets). 5 of 8 failures in CI Stress run 26364109146
were 15 s timeout trips across 5 different fixtures - the pattern that
per-fixture overrides in PRs #395, #397, #399 chased without converging.

Local timing measurements (documented in INVESTIGATION.md) show the
Framerate.* family completing in ~7 s on a dev box; CI VMs under
contention run 2-4x slower, putting fixture work at ~28 s on the worst
tick. 30 s matches the explicit override budget already used by four
fixtures and leaves a clean signal gap below the 60 s
HangWatchdogLoop dump-on-hang threshold.

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

Author: codemonkeychris

tests/Reactor.AppTests.Host/SelfTest/SelfTestFixtureBase.cs

@@ -9,7 +9,15 @@ internal abstract class SelfTestFixtureBase
 
     protected SelfTestFixtureBase(Harness harness) => H = harness;
 
-    public virtual TimeSpan FixtureTimeout => TimeSpan.FromSeconds(15);
+    // 30 s default chosen empirically from stress-run timing data: the
+    // Framerate.* fixture family completes in ~7 s on a dev box, and
+    // CI VMs under contention run 2-4x slower, putting fixture work at
+    // ~28 s on the worst tick. Previously several fixtures had explicit
+    // 30 s overrides (PR #397, #399); raising the default folds them in
+    // and prevents new sister fixtures from inheriting a budget that
+    // doesn't survive CI variance. The host-level HangWatchdogLoop at
+    // 60 s (SelfTestRunner.cs) continues to catch true dispatcher hangs.
+    public virtual TimeSpan FixtureTimeout => TimeSpan.FromSeconds(30);
 
     public abstract Task RunAsync();
 }