feat: PER-7348 add waitForReady() call before serialize()

[Shivanshu-07]

Summary

Adds Pattern C (in-browser) readiness gate ahead of PercyDOM.serialize() per PER-7348.

Pairs with: percy/cli#2184

Behavior

• Before serialize, the SDK calls PercyDOM.waitForReady(readinessConfig) — async — and awaits the promise.
• typeof PercyDOM.waitForReady === 'function' guard preserves backward compatibility with older CLI builds that don't ship the readiness API.
• readiness.preset === 'disabled' short-circuits — waitForReady is not called.
• If waitForReady rejects, the SDK logs at debug level and proceeds to serialize() so snapshot capture is never blocked.
• Diagnostics returned by waitForReady are attached to domSnapshot.readiness_diagnostics so the CLI can log timing and pass/fail.
• Readiness config resolves from per-snapshot options.readiness first, then utils.percy.config.snapshot.readiness from .percy.yml.

PercyDOM.serialize() itself is unchanged — still synchronous, called with the same options as today.

SDK-visible contract

// New — readiness step (skipped on old CLI / disabled preset)
if (preset !== 'disabled' && typeof window.PercyDOM?.waitForReady === 'function') {
  diagnostics = await window.PercyDOM.waitForReady(config);
}

// Unchanged
let domSnapshot = window.PercyDOM.serialize({ ...options });
if (diagnostics) domSnapshot.readiness_diagnostics = diagnostics;

Test plan

QUnit acceptance tests added to tests/acceptance/index-test.js under a readiness gate (PER-7348) module:

• waitForReady is called before serialize when CLI exposes it
• Backward compat — only serialize runs when waitForReady is absent
• readiness.preset: disabled skips waitForReady
• When waitForReady rejects, serialize still runs
• Diagnostics returned by waitForReady are attached to domSnapshot.readiness_diagnostics

⚠️ Unit tests written but not executed locally (missing ember-cli toolchain in the fan-out environment). CI will exercise them.

Smoke test against example-percy-ember was not run as part of this fan-out.

🤖 Generated with Claude Code