Chromium plugin: lazy loading for large DOM trees

[asklar]

Summary

Very large DOM trees are slow/expensive to serialize fully. Add lazy/bounded loading (docs/chromium-plugin.md → Future work).

Scope

• Respect --depth and add bounded/on-demand expansion (expand a specific subtree by element id — ties into --element and the proposed --query).
• Consider streaming or capped node counts with a clear ""truncated"" marker.
• Update docs/chromium-plugin.md.

Acceptance criteria

• Large pages return quickly with a bounded tree and an explicit truncation/expansion mechanism.
• Tests for depth/count limiting.

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Filed as part of turning README/docs "Future work" into tracked issues. See docs/architecture.md and CONTRIBUTING.md (""Adding a new provider"") for conventions.