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/tests/logger.test.ts

@@ -0,0 +1,52 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { createLogger } from '../core/logger';
+
+describe('logger', () => {
+  let writes: string[];
+  let write: (s: string) => boolean;
+
+  beforeEach(() => {
+    writes = [];
+    write = (s: string) => {
+      writes.push(s);
+      return true;
+    };
+  });
+
+  it('emits a JSON line per call with level and msg', () => {
+    const log = createLogger({ runId: 'r1', write });
+    log.info('hello');
+    expect(writes).toHaveLength(1);
+    const parsed = JSON.parse(writes[0]);
+    expect(parsed.level).toBe('info');
+    expect(parsed.msg).toBe('hello');
+    expect(parsed.runId).toBe('r1');
+    expect(parsed.timestamp).toBeTypeOf('string');
+  });
+
+  it('merges meta into the log line', () => {
+    const log = createLogger({ runId: 'r1', write });
+    log.warn('slow', { turnMs: 3000 });
+    const parsed = JSON.parse(writes[0]);
+    expect(parsed.turnMs).toBe(3000);
+    expect(parsed.level).toBe('warn');
+  });
+
+  it('child logger inherits parent meta', () => {
+    const log = createLogger({ runId: 'r1', write });
+    const roomLog = log.child({ room: 'room-1' });
+    roomLog.info('game_start');
+    const parsed = JSON.parse(writes[0]);
+    expect(parsed.room).toBe('room-1');
+    expect(parsed.runId).toBe('r1');
+  });
+
+  it('respects minimum level', () => {
+    const log = createLogger({ runId: 'r1', write, minLevel: 'warn' });
+    log.debug('nope');
+    log.info('nope');
+    log.warn('yes');
+    log.error('yes');
+    expect(writes).toHaveLength(2);
+  });
+});