Let mutations take subtree geometry up front and paint it in bands

[trissim]

Abandoned Codex attempt. Closed without merging.

This change adds Compositor.acquire_subtree_geometry(root), which returns a complete captured geometry before a write, or None when the placement is missing or incomplete. Compositor.render_subtree_strips now returns the size and an iterator of strip bands, each no taller than the captured viewport. Screen passes the captured geometry for each mutation root, and releasing a mutation re-checks pending layout work. It changes _compositor.py and screen.py, adds a notes file and test output under evidence/, and updates the compositor, paint and geometry tests.

[trissim]

Closing: this draft conflicts with main, and the compositor and screen layout code it reworks has since been rewritten there (stepped layout, scoped reflow, publication fixes). The branch tip is kept under refs/archive/20261011/heads/.