fix(client): skip structural whitespace in annotations

[auspham]

Summary

Prevent persistent text annotations from wrapping whitespace-only slices when a selection crosses nested elements. Those synthetic <mark> nodes became real grid or flex children in artifact layouts, producing large empty elements beside the selected content.

Add a browser regression that anchors a full card inside a CSS grid and verifies the selected text remains marked without adding whitespace marks or synthetic grid children.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor or tooling
• Other:

How was this tested?

The browser regression failed before the fix with five whitespace-only marks, then passed after wrapRange skipped whitespace-only text slices.

• python -m pytest tests/python
• ruff check skills/htmlit tests/python
• npm test (browser client)

Checklist

• I added or updated tests for this change (a bug fix has a test that fails before the fix).
• I did not add any runtime dependency to the skill (standard library only).
• I did not commit vendored assets (Mermaid, highlight.js, Idiomorph).
• Commits follow Conventional Commits and are signed off (git commit -s).