feat(session): default JSONL writes to packed rows

scripts/session-fixture-layout.snapshot.ts

@@ -0,0 +1,17 @@
+/** Repository-wide canonical-layout check for committed session fixtures. */
+
+import { resolve } from 'node:path'
+import { expect, it } from 'vitest'
+import { inspectSessionFixtureLayouts } from './session-fixture-layout.ts'
+
+const root = resolve(import.meta.dirname, '..')
+
+it('keeps every session-format JSONL fixture in canonical packed layout', () => {
+  const nonCanonical = inspectSessionFixtureLayouts(root)
+    .filter(fixture => fixture.source !== fixture.canonical)
+    .map(fixture => fixture.path)
+  expect(
+    nonCanonical,
+    'Run `pnpm run migrate:packed-session-fixtures` and commit the mechanical fixture rewrite.',
+  ).toEqual([])
+})