feat(soak): structured JSONL logger with child contexts

Single file, no transport, writes one JSON line per call to stdout.
Child loggers inherit parent meta so scenarios can bind room/game
context once and forget about it.

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

tests/soak/core/logger.ts

@@ -0,0 +1,59 @@
+/**
+ * Structured JSONL logger for the soak harness.
+ *
+ * One JSON line per call, written to stdout by default. Child loggers
+ * inherit parent meta so scenarios can bind room/game context once and
+ * every subsequent call carries it automatically.
+ */
+
+import type { Logger, LogLevel } from './types';
+
+const LEVEL_ORDER: Record<LogLevel, number> = {
+  debug: 0,
+  info: 1,
+  warn: 2,
+  error: 3,
+};
+
+export interface LoggerOptions {
+  runId: string;
+  minLevel?: LogLevel;
+  /** Defaults to process.stdout.write bound to stdout. Override for tests. */
+  write?: (line: string) => boolean;
+  baseMeta?: Record<string, unknown>;
+}
+
+export function createLogger(opts: LoggerOptions): Logger {
+  const minLevel = opts.minLevel ?? 'info';
+  const write = opts.write ?? ((s: string) => process.stdout.write(s));
+  const baseMeta = opts.baseMeta ?? {};
+
+  function emit(level: LogLevel, msg: string, meta?: object): void {
+    if (LEVEL_ORDER[level] < LEVEL_ORDER[minLevel]) return;
+    const line = JSON.stringify({
+      timestamp: new Date().toISOString(),
+      level,
+      msg,
+      runId: opts.runId,
+      ...baseMeta,
+      ...(meta ?? {}),
+    }) + '\n';
+    write(line);
+  }
+
+  const logger: Logger = {
+    debug: (msg, meta) => emit('debug', msg, meta),
+    info: (msg, meta) => emit('info', msg, meta),
+    warn: (msg, meta) => emit('warn', msg, meta),
+    error: (msg, meta) => emit('error', msg, meta),
+    child: (meta) =>
+      createLogger({
+        runId: opts.runId,
+        minLevel,
+        write,
+        baseMeta: { ...baseMeta, ...meta },
+      }),
+  };
+
+  return logger;
+}