M8.4: Liquify keeps going while held, leaves non-paths alone, strokes apply incrementally

Holding the brush still keeps Twirl, Pucker and Bloat applying, scaled by the
time held: a new Tool::tick (time supplied by the host: canvas frames, or
`holdMs` on ui.pointer / pointer_gesture events) repeats the last point every
0.1 s, one more dab there. Liquify never touches graphs, meshes, type, symbols,
images, or envelopes, repeats and blends with their contents, nor guides in
selection mode; those under the brush come back in `skipped` with a `warning`
that the status bar shows (UiRequest::Status). A drag applies only the dabs
each new sample adds (the stroke's state is kept between previews), giving the
same result as the whole stroke, so journal replay still reproduces it.

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

docs/control-protocol.md

@@ -489,6 +489,11 @@ then the tool's options `detail`, `simplify` and `simplifyOn` (Warp, Twirl, Puck
 `vertical` (Wrinkle, %), and `showBrush`. `ui.dialog.confirm` runs `tool.setOption {tool, values}`. `ui.pointer`
 events take `pressure` (0..1, default 1): it is the Liquify intensity while Use Pressure Pen is on.
 
+`ui.pointer` events also take `holdMs` (0..60000): the pointer then holds still that long, button down, before the
+next event. Twirl, Pucker and Bloat keep applying while held (a repeat of the last point every 0.1 s), exactly as
+for the same time held with the mouse. Liquify leaves type, symbols, images, graphs, meshes, envelopes, repeats and
+blends as they are and says so in the status bar.
+
 Perspective grid presets: in the `perspectiveGrid` dialog, `name` is the preset whose fields are loaded (setting the
 fields of a preset, as the Preset menu does, keeps its name; other changes clear it, which reads [Custom]). Save
 Preset… sets `__mode: "save"` and `__from: "define"` with `name` the new preset's name: `ui.dialog.confirm` then saves