Adds --check (compile every effect without camera or window), EFFECTS.md
(the contract for writing effects) and AGENTS.md. The POC stays available
at the 'poc' tag.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Snapshot before the Python POC is removed. The POC (main.py, effects.py,
panel.py, shaders.py, effects/*.glsl, flake.nix) is the reference for the
native app under native/.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>