Cumulative path/pattern calculations

For a sequence of X/Y deltas:

1. Start cumulative X/Y at zero.

2. Add each delta once to compute path points.

3. Scan the entire path for min/max bounds.

4. Derive a display scale from the largest span.

5. Render full path, current prefix, or highlight overlays without mutating the source data.

For edits:

• per-step edit changes one stable record and one index;

• whole-pattern strength transforms every selected point;

• highlighted-range strength transforms only the bounded selection;

• clamp every result to its storage type/range;

• mark dirty but do not commit until save.