Sample only a sub-rectangle of the captured frame instead of the whole display,
so a taskbar, game HUD, or letterbox bars don't pollute the border colours — the
first functional gap a reviewer hits (capture was full-display only).
- New pure crop_screen_capture() returns a numpy view (no copy), fast-paths the
full-frame case, and clamps degenerate/out-of-range ROIs to >=1px.
- ROI lives on CalibrationConfig (simple mode) as fractions 0..1 with a has_roi
helper; applied in the picture color-strip stream just before border
extraction, clamping border_width to the cropped size. Additive + backward
compatible (full-frame default, omitted from serialization when unset -> no
migration).
- Round-trips through the calibration schema automatically; frontend adds an
X/Y/Width/Height (%) 'Capture region' group to the calibration editor with
i18n (en/ru/zh).
10 unit tests (crop geometry, view-not-copy, clamping, ROI round-trip, legacy
default); full suite green (1946 passed).
