feat(soak): populate scenario + scenario registry

Partitions sessions into N rooms, runs gamesPerRoom games per room
in parallel via Promise.allSettled so a failure in one room never
unwinds the others. Errors roll up into ScenarioResult.errors.

Verified via tsx: listScenarios() returns [populate], getScenario()
resolves by name and returns undefined for unknown names.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/soak/scenarios/index.ts

@@ -0,0 +1,22 @@
+/**
+ * Scenario registry — name → Scenario mapping.
+ *
+ * Runner looks up scenarios by name. Add a new scenario by importing
+ * it here and adding an entry to `registry`. No filesystem scanning,
+ * no magic.
+ */
+
+import type { Scenario } from '../core/types';
+import populate from './populate';
+
+const registry: Record<string, Scenario> = {
+  populate,
+};
+
+export function getScenario(name: string): Scenario | undefined {
+  return registry[name];
+}
+
+export function listScenarios(): Scenario[] {
+  return Object.values(registry);
+}