Visual compare can screenshot before local stylesheets finish loading

## Problem

`wordpress.visual-compare` can produce and classify a pixel diff while local linked stylesheets are still pending. `waitForVisualComparePaintReady()` caps its load wait and stylesheet wait at 10 seconds each, even when the command has a substantially larger wall timeout. Capture diagnostics then report pending stylesheets and low-confidence readiness, but the already-written screenshots continue into comparison as if they were conclusive.

This can create a false large layout delta when a pending stylesheet owns responsive visibility or geometry.

## Expected behavior

- Use a bounded stylesheet-readiness budget derived from the visual command timeout.
- Preserve enough wall time for settling, snapshots, screenshots, and diagnostics.
- Do not classify screenshots as conclusive visual evidence when required local stylesheets remain incomplete.
- Cover delayed local stylesheet loading with deterministic tests.

## Acceptance criteria

1. A local stylesheet that completes within the derived readiness budget is applied before capture.
2. A stylesheet that remains pending after the budget produces explicit incomplete-capture evidence rather than a misleading visual-diff classification.
3. Existing external-request isolation and overall command bounds remain intact.

## AI assistance

OpenAI GPT-5.6 Sol via OpenCode diagnosed the capture evidence, traced the owning implementation, and drafted this issue under human direction.

Start at waitForVisualComparePaintReady() and trace the wordpress.visual-compare capture path, including diagnostics, settling, snapshots, and screenshots. Exercise delayed local stylesheet loading with deterministic tests; done means readiness uses the command timeout without breaking external-request isolation, and pending stylesheets produce explicit incomplete-capture evidence instead of a conclusive visual-diff classification.

Written by the indexing model from the issue text.