refactor(cli): unify seek/settle and Chrome launch across browser commands

seekCompositionTimeline becomes the single seek implementation with
per-caller settle options (rAF mode, font wait, settle sleep), replacing
the divergent local seekTo copies in validate and layout. All three
launch paths now build args via the engine's buildChromeArgs; screenshot
paths keep the engine's software-GPU default for deterministic output.

inspect gains one transient content_overlap warning on product-promo
(t=12.22s): the gsap.ticker.tick flush samples timeline state the old
layout seek missed.

packages/cli/src/commands/validate.test.ts

@@ -131,6 +131,16 @@ describe("waitForPreferredSeekTarget", () => {
 
     await expect(waitForPreferredSeekTarget(page, 1)).resolves.toBeUndefined();
   });
+
+  it("does not fail validation when the page stub throws synchronously", async () => {
+    const page = {
+      waitForFunction: vi.fn(() => {
+        throw new Error("waiting failed synchronously");
+      }),
+    };
+
+    await expect(waitForPreferredSeekTarget(page, 1)).resolves.toBeUndefined();
+  });
 });
 
 describe("extractCompositionErrorsFromLint", () => {