Wait for complete hover-card rendering

[brsbl]

Summary

• Adds an explicit hover-card render state so screenshot and UI automation can wait for every asynchronous section, including timing and pull-request metadata.
• Marks a card complete only after async enrichment, fonts, images, and two paint frames have settled; error states remain explicit.
• Preserves the existing hover timing and visual presentation.

Rendering evidence

A cold real-BB render completed in 337 ms: summary 182 ms, timing 41 ms, and pull-request metadata 43 ms. No rendering bottleneck was found, so this PR intentionally makes no speculative performance change.

The matched screenshot fixture delays pull-request metadata by 1.5 seconds. The parent can be captured before the PR section arrives; the branch stays busy until the complete card includes #67 and Merged.

Before — parent captured too early	After — complete readiness

Validation

• npm run check --workspace=bb-plugin-thread-hover-cards
• git diff --check
• Real BB dev app at 1440x900 with the same project, thread, viewport, window bounds, and delayed PR fixture on both revisions

BB-Thread-ID: thr_bwikffmsvp