Spiral tool: Alt-drag adds or removes segments, and Ctrl/Cmd-drag changes the decay

The reference app adds or subtracts winds from a spiral with Alt-drag
and changes its decay rate with Ctrl/Cmd-drag. VectorCraft's Spiral
tool did nothing with either key during a drag.

While a spiral is dragged out, Alt or Ctrl/Cmd keeps the spiral's
inner end at the distance from the center it had when the key went
down, and the outer end follows the pointer. With Alt the number of
segments changes to the nearest whole number that fits, within 2 to
1000, so dragging outward adds segments and dragging inward removes
them. With Ctrl/Cmd the decay changes to fit, within 5 to 99.99
percent, the decays a spiral is drawn with. When both keys are held,
Ctrl/Cmd changes the decay. Up/Down with a key still held move the
held radius to the new segment count. The new values stay when the
key is let go, and as the Spiral tool's segments and decay options
for the next spiral. Both keys also work with a 5 percent decay and
hundreds of segments.

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

docs/mcp.md

@@ -486,6 +486,17 @@ values, and they stay as the Arc tool's `closed`, `baseAlong` and `slope` option
 dragged, `c` chooses the tool bound to C, and `f` runs the shortcut bound to F in remote mode (headless has no
 binding for it).
 
+While a spiral is being dragged out with the Spiral tool, a `drag` event with `mods.alt` or `mods.cmd` keeps the
+spiral's inner end at the distance from the center it had at the first such event, and the outer end follows the
+pointer. With `mods.alt` the number of segments changes to the nearest whole number of segments that fits (within
+2…1000), so dragging outward adds segments and dragging inward removes them; with `mods.cmd` the decay changes to fit
+(within 5…99.99 %). The spiral drawn on release uses the new values, and they stay as the Spiral tool's `segments` and
+`decay` options for the next spiral:
+
+```json
+{"name":"pointer_gesture","arguments":{"tool":"spiral","events":[{"kind":"down","x":100,"y":100},{"kind":"drag","x":200,"y":100,"mods":{"alt":true}},{"kind":"drag","x":300,"y":100,"mods":{"alt":true}},{"kind":"up","x":300,"y":100}]}}
+```
+
 Appearance stacks: an object can carry several fills and strokes (`appearance.addFill`, `appearance.addStroke`), indexed
 in paint order (0 is painted first, the bottom row of the Appearance panel). Cmd+/ runs `appearance.addFill` and
 Cmd+Alt+/ runs `appearance.addStroke` (`press_key {key: "/", mods: {cmd: true, alt: true}}`). `paint.setFill`, `paint.setStroke`,

docs/target-app-parity.md

@@ -673,6 +673,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - **Shapes:** all shape tools (including Flare) and the line, arc, spiral and grid tools. Space held while dragging out a shape, line, arc, spiral or grid moves it at its current size. The Flare tool's center drag turns the rays with the pointer, Shift keeps them in their original orientation (unturned, or turned by a nonzero General › Constrain Angle), and Space held during either of its drags moves the flare. While a grid is dragged out, Up/Down change its rows (concentric dividers) and Left/Right its columns (radial dividers). A click with a grid tool asks for the size, the divider counts and the grid options (#1377, in part): a skew for each set of dividers, Use Outside Rectangle As Frame or Create Compound Path From Ellipses, and Fill Grid; they are parameters of `shape.rectangularGrid` and `shape.polarGrid`, stay as the tool's options and apply to drags too. The curve a skew percentage follows and the direction of the radial skew are our own (the public documentation gives neither), and the dialogs have no reference point locator. An ellipse's Pie Start and End Angle and Invert Pie (Transform and Properties panels' Ellipse Properties, `object.setLiveShape {pieStart, pieEnd, invertPie}`) cut it as a pie (#812). With the bounding box showing, the Selection tool shows a selected ellipse's two pie widgets just outside it; dragging one sets its angle in one undo step and double-clicking one makes the ellipse whole again. A live polygon's Polygon Properties there: side count, angle, corner type and radius, radius, side length and Make Sides Equal once an uneven scale has made its sides differ (`object.setLiveShape {sides, polygonAngle, polygonRadius, sideLength, makeSidesEqual}`, part of #812); a star is a plain path, as in Illustrator. The Flare Tool Options last as the tool's options: the next flare dragged out uses them, a click's dialog shows them and double-clicking the tool opens them (part of #812).
   - **Shapes:** all shape tools (including Flare) and the line, arc, spiral and grid tools. Space held while dragging out a shape, line, arc, spiral or grid moves it at its current size. The Flare tool's center drag turns the rays with the pointer, Shift keeps them in their original orientation, and Space held during either of its drags moves the flare. While a grid is dragged out, Up/Down change its rows (concentric dividers) and Left/Right its columns (radial dividers). A click with a grid tool asks for the size, the divider counts and the grid options (#1377, in part): a skew for each set of dividers, Use Outside Rectangle As Frame or Create Compound Path From Ellipses, and Fill Grid; they are parameters of `shape.rectangularGrid` and `shape.polarGrid`, stay as the tool's options and apply to drags too. The curve a skew percentage follows and the direction of the radial skew are our own (the public documentation gives neither), and the dialogs have no reference point locator. An ellipse's Pie Start and End Angle and Invert Pie (Transform and Properties panels' Ellipse Properties, `object.setLiveShape {pieStart, pieEnd, invertPie}`) cut it as a pie (#812). With the bounding box showing, the Selection tool shows a selected ellipse's two pie widgets just outside it; dragging one sets its angle in one undo step and double-clicking one makes the ellipse whole again. A live polygon's Polygon Properties there: side count, angle, corner type and radius, radius, side length and Make Sides Equal once an uneven scale has made its sides differ (`object.setLiveShape {sides, polygonAngle, polygonRadius, sideLength, makeSidesEqual}`, part of #812); a star is a plain path, as in Illustrator. The Flare Tool Options last as the tool's options: the next flare dragged out uses them, a click's dialog shows them and double-clicking the tool opens them (part of #812).
     While an arc is dragged out, C switches it between open and closed, F switches its base between the X and Y axes in the same box, and Up/Down raise and lower its slope by 10 within −100…100; each press switches once, and the values stay as the Arc tool's options. The [default keyboard shortcuts](https://web.archive.org/web/20260726060015/https://helpx.adobe.com/illustrator/using/default-keyboard-shortcuts.html) (archived copy) list C, F and Up/Down for an arc being dragged out. F as the base switch, the arrows changing the slope, the step of 10 and one switch per press are our own rules.
+    While a spiral is dragged out, Alt adds or removes segments and Ctrl/Cmd changes the decay; both keep the spiral's inner end at the distance from the center it had when the key went down while the outer end follows the pointer, and the values stay as the Spiral tool's options. 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. The held inner radius, rounding to whole segments, the ranges (2…1000 segments, 5…99.99 % decay) and the new values staying are our own rules.
   - **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.
     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`).