Per-chunk split for the live-progress overlay (DD-006 §4.5).
Uncertainty band (loSeg < index <= hiSeg) — empty when loSeg === hiSeg.
Segments completed by the printer (IR index <= loSeg), within the layer range.
Remaining path (index > hiSeg) — the translucent ghost pass.
Per-chunk split for the live-progress overlay (DD-006 §4.5).