feat(sequencing): mute parts until a line or click sequences them in Sequence by Path

With every part drawn the same, it was hard to see which parts the drawn
lines had not reached yet. Parts no line reaches are now drawn muted, and
update live while dragging. Clicking a part adds it as a one-point line, so
a stray part can be picked without drawing a line to it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/automatic-scrap-cutoffs.md

@@ -45,7 +45,7 @@ Existing full-width vertical cut-offs inside the used span become fixed strip bo
 
 For **Plate > Sequence Parts > Manual Sequence**, choose the one-based sequence number and hover a part on the sheet; only the part under the pointer is outlined. Click it to place it at that exact position, then the number advances up to the last part. Clicking empty sheet space changes nothing. Use **Done**, the window close button, right-click on the sheet, or Escape to leave the tool. Manual reordering changes neither geometry nor quantities.
 
-For **Plate > Sequence Parts > Sequence by Path**, drag lines over the sheet with the left button in the order you want the parts cut. Lifting the button between lines is a rapid: the dashed jump to the next line's numbered start claims no parts. Each part belongs to the line crossing its bounding box that passes closest to its bounding-box center, and lines are cut in drawing order, each in the order its nearest points fall along it; where a line passes a part more than once, the closest pass decides. Parts no line crosses are cut after all the others, in the order they would follow their nearest line. The numbers update as you drag, and the plate is reordered when you release. **Undo Line**, Backspace or Ctrl+Z removes the last line; **Clear** or right-click removes them all and restores the order the tool started with. Cut-first parts stay ahead of the rest, and cutoffs follow the same dependency rules as Part Sequencing below. Escape mid-drag drops only that line; otherwise Escape, **Done** or closing the window leaves the tool.
+For **Plate > Sequence Parts > Sequence by Path**, drag lines over the sheet with the left button in the order you want the parts cut; clicking a part adds it as a one-point line that claims the parts whose bounding boxes hold the click. Lifting the button between lines is a rapid: the dashed jump to the next line's numbered start claims no parts. Each part belongs to the line crossing its bounding box that passes closest to its bounding-box center, and lines are cut in drawing order, each in the order its nearest points fall along it; where a line passes a part more than once, the closest pass decides. Parts no line crosses are cut after all the others, in the order they would follow their nearest line, and are drawn muted until a line or click reaches them. The numbers update as you drag, and the plate is reordered when you release. **Undo Line**, Backspace or Ctrl+Z removes the last line; **Clear** or right-click removes them all and restores the order the tool started with. Cut-first parts stay ahead of the rest, and cutoffs follow the same dependency rules as Part Sequencing below. Escape mid-drag drops only that line; otherwise Escape, **Done** or closing the window leaves the tool.
 
 After adding cutoffs, apply **Part Sequencing** to the current plate or all plates. Each cutoff is moved earlier as needed so it is cut before every part its nominal line passes through. Ordinary parts retain their relative order from the chosen sequencing route. A cutoff that splits the whole sheet — one continuous cut from sheet edge to sheet edge, such as a tail separator — goes after every part and every other cutoff, so the head never travels back across the separated edge. Cutoffs that cross no part but leave gaps keep their normal route position. Cutoff geometry, clearance, part programs, placements, and quantities are unchanged.