Appearance: Cmd+/ adds a new fill and Cmd+Alt+/ adds a new stroke

The reference app adds a new fill with Ctrl+/ on Windows and Command+/
on macOS, and it adds a new stroke with Ctrl+Alt+/ on Windows and
Option+Command+/ on macOS. VectorCraft had Add New Fill and Add New
Stroke on the Appearance panel, with no default key for either.

appearance.addFill now has the default Cmd+/ and appearance.addStroke
has Cmd+Alt+/, which are Ctrl+/ and Ctrl+Alt+/ on Windows and Linux.
Like the panel's buttons and menu items, each key runs its command with
no params and adds the item to every selected object. Keyboard Shortcuts
lists both under Window › Appearance with their keys, and press_key
runs them for agents.

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

crates/engine/src/cmd/appearance.rs

@@ -37,7 +37,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "appearance.addFill",
             "Add New Fill",
             ["Window", "Appearance"],
-            None,
+            Some("Cmd+/"),
             "{ids?|id?, target?: \"object\"|\"contents\"} add a fill on top of each selected object's own stack, on an image right below its Contents row (a copy of its top fill; type without one: its characters' fill; else the default fill)",
             has_doc,
             |s, p| add_item(s, p, true)
@@ -46,7 +46,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "appearance.addStroke",
             "Add New Stroke",
             ["Window", "Appearance"],
-            None,
+            Some("Cmd+Alt+/"),
             "{ids?|id?, target?} add a stroke on top of each selected object's own stack, on an image right below its Contents row (a copy of its top stroke, else the default stroke)",
             has_doc,
             |s, p| add_item(s, p, false)