Start heatmap loads before score results

[adrunkhuman]

Summary

• Start loading /heatmap from htmx:beforeRequest using the current preference form values.
• Keep POST /score as the ranked-results request and render those results when the HTMX response arrives.
• Add frontend runtime coverage that proves the heatmap load starts before the /score response is rendered.

Testing

• Commit hooks: ruff check, ruff format, ty type checker, pytest
• uv run pytest tests/test_app_frontend.py tests/test_app_shell.py -q
• uv run ruff check frontend tests/test_app_frontend.py

Notes

• This is an experiment for VPS testing.
• It may duplicate score-field work when GET /heatmap beats POST /score to the backend cache.
• The thing to measure is whether faster heatmap start time is worth the possible extra CPU under slider churn.

[adrunkhuman]

No visible win at all. Site usage feels the same. Not worth pursuing for now.