Knife: Alt cuts in a straight line and Shift+Alt cuts at 45° steps

The Knife cut along the freehand drag whatever keys were held. The
reference app cuts in a straight line with Alt-drag, and it cuts at 45
or 90 degrees with Shift+Alt-drag.

With Alt held at the press, or pressed before the drag has moved, the
Knife's stroke is the press point and the pointer: the dashed line
shows it while dragging, and the release sends path.knife with those
two points. With Shift held too, the line keeps to 45° steps from the
Constrain Angle, as the Pencil's Shift line does. Shift is read on each
drag event and at the release. The cut stays straight when Alt is let
go, a line taken back to its press point sends no command, and Shift
alone leaves the cut freehand.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -676,6 +676,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
     While a spiral is dragged out, Alt adds or removes whole winds of four segments and Ctrl/Cmd changes the decay. The spiral's inner end keeps the distance from the center it had on the first modified drag event, while the outer end follows the pointer. The [default keyboard shortcuts](https://web.archive.org/web/20260726060015/https://helpx.adobe.com/illustrator/using/default-keyboard-shortcuts.html) (archived copy) list Alt-drag to add or subtract winds and Ctrl-drag to change the decay rate; [Draw spirals](https://helpx.adobe.com/illustrator/desktop/draw-shapes-and-paths/draw-shapes/draw-spirals.html) defines four segments per full wind. Rounding the fitted count to whole winds from the count on the first Alt-only drag event, the held inner radius, the ranges (2…1000 segments, 5…99.99 % decay), Ctrl/Cmd taking precedence when both are held, and the new values staying as Spiral tool options are our own rules. Switching back to Alt uses the current count as its new base while keeping the held inner end.
   - **Cutting:** Eraser, Scissors, Knife, Mirror & Cut, Line Cut and Rectangle Cut (real geometry, compound paths keep their holes).
     While Alt is held, the Scissors tool works as the Add Anchor Point tool ([Default keyboard shortcuts](https://web.archive.org/web/20260726060015/https://helpx.adobe.com/illustrator/using/default-keyboard-shortcuts.html)): a click on a segment adds an anchor point without cutting (`path.insertAnchor`), and the pointer shows the Add Anchor Point cursor. When Alt is let go, a click cuts again.
+    Alt-drag with the Knife cuts in a straight line, and Shift+Alt-drag cuts at 45° or 90°, as [Cut objects using the Knife and Scissors tools](https://web.archive.org/web/20260926095638/https://helpx.adobe.com/illustrator/desktop/manage-objects/edit-objects/cut-objects.html) and [Default keyboard shortcuts](https://web.archive.org/web/20260726060015/https://helpx.adobe.com/illustrator/using/default-keyboard-shortcuts.html) describe. With Alt held at the press, or pressed before the drag has moved, the dashed line runs from the press point to the pointer and the release sends `path.knife` with those two points. With Shift held too, the line keeps to 45° steps from the Constrain Angle. The cut stays straight when Alt is let go, and a line taken back to its press point sends no command.
     Eraser Tool Options has Angle and Roundness besides Size, as [Erase paths using the Eraser tool](https://helpx.adobe.com/illustrator/desktop/draw-shapes-and-paths/modify-paths/erase-paths-using-eraser-tool.html) describes: the Eraser's tip is an ellipse Size across, its long axis at Angle and its short axis Roundness percent of the long one, and a drag with a tip that is not round shows the outline of the area it erases (`path.eraseRegion` `angle` and `roundness`).
   - **Transform:** Rotate, Reflect, Scale, Shear (click or Alt-click snaps the reference point to anchors and centres; Alt-click opens the tool's dialog, and a drag with Alt down at the release, from the press or pressed mid-drag, transforms a copy), Reshape, Free Transform (free and perspective distort and shear, switched by the keys held during a handle drag).
     Align, Distribute, Distribute Spacing, Reflect and Shear return an error for any `horizontal`, `vertical`, `to`, `axis`, `bounds` or `spacing` value they don't take, listing the choices, and Align returns one when a call gives neither `horizontal` nor `vertical`; nothing moves and no undo step is recorded. Align and Distribute act on the anchor points selected with Direct Selection: the points of one path align to each other, not to the artboard, and Distribute spaces them (#1641, fixes #1636; aligning to a key anchor is still missing).