M1: paint depth: gradient stops on the canvas, colour models, colours on this page (#7)

- Gradient stops on the canvas: drag a stop along the gradient line, click the line to add a
  stop (sampled colour), the selected stop is ringed and shared with the picker; Delete removes
  it (two always stay).
- Colour picker value row: a model dropdown (Hex, RGB, HSL, HSB, CSS) with its fields and the
  alpha field; scrubbing works on the numeric fields.
- "On this page" swatches: the page's most used solid colours (cached per document snapshot);
  swatches set a solid colour or the selected gradient stop.
- New command `paint.documentColors {page?, limit?}`; control channel `ui.set {colorPicker}`.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -88,10 +88,11 @@ text tool).
 
 Invoke a menu item exactly like clicking it (this may open a dialog). Returns `{ok, lastError}`.
 
-### `ui.set {rightTab?, leftTab?, theme?, home?, palette?, mainMenu?, layerFilter?, collapseLayers?}`
+### `ui.set {rightTab?, leftTab?, theme?, home?, palette?, mainMenu?, colorPicker?, layerFilter?, collapseLayers?}`
 
 Switch panels and modes: `rightTab` `design|prototype|inspect`, `leftTab` `file|assets`, `theme`,
-`home` (bool), `palette` (bool: the Actions palette), `mainMenu` (bool), `layerFilter` (string:
+`home` (bool), `palette` (bool: the Actions palette), `mainMenu` (bool), `colorPicker`
+(`{fills?, index?}`: open the colour picker on the selection's paint; `null` closes it), `layerFilter` (string:
 open the layers search with that filter; `null` closes it), `collapseLayers` (true: collapse the
 layers tree). Returns `ui.inspect`.