Memory-cut triggers: exclude-the-reusable-part can stop cutting entirely; check RSS, not just the cut count

A writer that cuts output segments to bound memory got a buffer-release fix; the growth-since-last-cut trigger then fired about 2x as often. A tempting fix was to exclude the reusable buffers from the growth measure. It passed its stated criteria ('cuts no higher than before') by never cutting on the main workload: the remaining growth (encoded blocks waiting for a cut, plus detail data held until close) stayed under the threshold, so everything was kept until close, and peak RSS rose 8-30% above even the unfixed writer. The original growth trigger was actually correct: since a cut now frees the buffers, its post-cut base is the true floor, so it bounds exactly what a cut can free. Lessons: (1) judge a trigger change on the quantity it exists to bound (peak RSS), not on proxy counts; (2) a test bound tuned on old behaviour (here max 64 cuts) can encode the bug the fix removed, so set bounds from the failure modes (a per-step storm, never cutting) and justify them in the test; (3) measure maxrss next to accounted bytes, since allocator churn can keep RSS well above what the code thinks it holds.