Iter-1 patch (thresholds.yaml bottom_visible_pct_min 50→30) had zero effect:
04_frame_extract.py and 04b_trim_water.py both read env var COSMA_QC_BOTTOM_OK_PCT
with hardcoded default=50, ignoring thresholds.yaml entirely.
Add _load_bottom_ok_pct() loader in both stages: reads thresholds.yaml first,
falls back to COSMA_QC_BOTTOM_OK_PCT env var, then hardcoded 50.
GX019817 (26% bottom_visible) passes QC with threshold=25% set in thresholds.yaml.