MCP: select.key checks its id, and agents can read the key object

Agents could set and clear the key object but not read it:
document.inspect, the control channel's ui.pointer reply and the
headless pointer_gesture reply left it out, and select.key returned
null. select.key with an id that was not selected, or with a value
that is not an id, such as "x", cleared the key without an error. A
compound-path member or an object inside a selected group could become
the key.

select.key now checks its id as select.set, select.add and
select.toggle do (#784). It returns an error for an id that is not a
selected top-level object and leaves the key as it was. Layers,
compound-path members and objects inside another selected object are
not top-level objects. select.key {id: null} used to clear the key and
is now an error; select.key {} still clears it. select.key returns
{key}, and document.inspect and both pointer replies include the key
object as "key".

In isolation mode a Selection-tool click can pick an object that
select.key does not accept as the key: a sublayer inside an isolated
layer, a member inside an isolated compound path, or an object in an
isolated group that is also selected. The tool no longer sends
select.key {id} for such an object. When the object is already the
key, the tool still sends select.key {}, which clears the key.

A recorded action whose select.key step names an object that is not a
selected top-level object when the action plays now fails and is
rolled back, as an action with a select.set step naming a deleted
object has failed since #784.

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

docs/control-protocol.md

@@ -55,12 +55,12 @@ the two can still win that race.
 |---|---|---|
 | `engine.execute` | `{command, params}` | run any engine or UI command (see `engine.commands`) |
 | `engine.commands` | | every command with label, shortcut, params doc, enablement |
-| `document.inspect` | `{depth?, childLimit?}` | layer tree, selection, history, paint defaults; the options slice the layer tree as `document.node {summary: true}` does |
+| `document.inspect` | `{depth?, childLimit?}` | layer tree, selection, the key object (`key`, null without one), history, paint defaults; the options slice the layer tree as `document.node {summary: true}` does |
 | `ui.inspect` | | tool, UI state, `screenMode` (0 normal, 1 full screen with menu bar, 2 full screen, 3 Presentation Mode), `taskBar` (`pinned`, and `rect` `[x, y, w, h]` while the Contextual Task Bar shows), `freeTransformWidget` (`[x, y, w, h]` while the Free Transform widget shows), view, canvas rect, window size, perf, background saves and exports still running, `nativeMenuBar` (true when the menus are in the macOS menu bar rather than the window; `ui.menu.list` lists the in-window menus either way), `fileDialog` (`"open"`, `"place"`, `"save"` or `"folder"` while a native file dialog is open, `null` otherwise; see below) |
 | `ui.menu.list` / `ui.menu.invoke` | `{command, params}` | the full menu tree / invoke an item |
 | `ui.contextMenu.list` | | the canvas context menu for the current selection, flattened like `ui.menu.list` (`path` holds its submenus). `ui.click {x, y, button: "right"}` on the canvas opens it, after selecting the object there unless it is already selected |
 | `ui.tool.select` / `ui.tool.list` | `{tool}` | |
-| `ui.pointer` | `{events:[{kind: down|drag|up|move|doubleclick, x, y, space?: "doc"|"screen", mods?}]}` | drive the active tool exactly like the mouse: `mods.cmd` held at a press with any tool but a selection tool drags with the selection tool chosen last (Direct Selection with the Pen until one is), and the release gives the tool back as it was |
+| `ui.pointer` | `{events:[{kind: down|drag|up|move|doubleclick, x, y, space?: "doc"|"screen", mods?}]}` | drive the active tool exactly like the mouse: `mods.cmd` held at a press with any tool but a selection tool drags with the selection tool chosen last (Direct Selection with the Pen until one is), and the release gives the tool back as it was; the reply lists `selection`, `key` (the key object, null without one) and `tool` |
 | `ui.key` / `ui.text` | `{key, shift?, alt?, cmd?}` / `{text}` | synthetic keyboard input |
 | `ui.wheel` | `{x, y, dy?, dx?, unit?: "line"\|"point", shift?, alt?, cmd?}` | a mouse wheel turn over screen point (x, y): `dy` notches up (+) or down, `dx` sideways. Over the canvas the wheel scrolls and Cmd- or Alt-wheel (Option on the Mac) zooms about the pointer; with the `zoomWithMouseWheel` preference the wheel and Alt-wheel zoom about the pointer, Shift-wheel scrolls up and down and Cmd-wheel (Ctrl on Windows and Linux) sideways. Over a focused numeric field (click it first) each notch steps its value as Up/Down do (Shift: ten, Cmd/Ctrl: a tenth) and the panel stays put; over anything else in a panel the wheel scrolls it |
 | `ui.set` | `{brightness?, panel?, rulers?, outline?, grid?, smartGuides?, boundingBox?, controlBar?}` | |