Add HTML brainstorm reports

[arsis-dev]

Summary

Adds static HTML companion reports for Open Collider brainstorm outputs.

The existing Markdown reports remain unchanged as the text/archive format. This PR adds browser-readable ITER_REPORT.html and REPORT.html files generated from the same structured run artifacts.

What changed

• Generate iter_NNN/ITER_REPORT.html alongside iteration Markdown reports.
• Generate brainstorm_NNN/REPORT.html alongside aggregate Markdown reports.
• Render summary metrics, curated ideas, insights without collision, and the raw retained pool.
• Keep curated output first, insights second, and raw retained ideas as a lower-priority audit trail.
• Preserve flagging state and display numbering when numbering_map.json is present.
• Escape generated content before rendering HTML.
• Avoid repeating the card title inside the card body.

Why

Markdown is useful for archival output, but long brainstorm runs are hard to scan and review in plain text. The HTML reports make curation and post-run review easier without changing the underlying data model or adding dependencies.

Tests

PYTHONPATH=src .venv/bin/python -m pytest tests/ -v