Complete reference for per-recipe CI customization: all 18 recipe_meta keys
(incl. the base pin UPGRADE_BASE_VERSION), the six divergent meta loaders,
every hook file (test_<op>.py overlays, ops.py, install_steps.sh,
setup_custom_tests.sh, compose.ccci.yml), env contracts, and §8 known
limitations / restructuring candidates (R1 loader drift, R2 dead SCREENSHOT
knob, R6 silent-typo hazard, ...). Written for operator review ahead of a
possible restructure.
