feat(hydration): Add a hydrationStarted lifecycle callback

[radium-v]

Pull Request

📖 Description

Adds a hydrationStarted lifecycle callback to HydrationControllerCallbacks and fixes the timing of hydration lifecycle events. Previously, elementWillHydrate fired during the queuing/defer-hydration phase rather than at the point of actual hydration. This PR restructures the hydration pipeline so that:

• hydrationStarted fires once, immediately before the first element actually begins hydrating
• elementWillHydrate and elementDidHydrate fire inline with the hydration work, not during registration

Also expands the performance metrics test fixture and adds cross-browser Playwright tests (Chromium, Firefox, WebKit) to validate the full hydration lifecycle ordering.

👩‍💻 Reviewer Notes

• The HydratableElementController previously used private notifyWillHydrate, notifyDidHydrate, and notifyHydrationComplete methods that wrapped callbacks in try/catch. These have been inlined to reduce indirection — the try/catch blocks remain.
• The hydrationStarted callback uses a static boolean guard so it fires exactly once per hydration cycle, resetting when hydrationComplete runs.
• The performance metrics fixture uses detail.sequence on marks to verify ordering even when performance.now() values collide (common in WebKit due to timer coarsening).

📑 Test Plan

• 11 Playwright tests across 3 browsers (Chromium, Firefox, WebKit) validate the full lifecycle ordering: element-register → template-update → element-define → hydration:started → per-element hydrations → hydration:complete
• Tests use monotonic sequence counters as tiebreakers for timestamp collisions
• All existing tests pass

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

⏭ Next Steps

• Consider exposing the sequence counter pattern as a first-class utility for lifecycle instrumentation