feat(ui): add momentary keyboard control surface

docs/KEY_BUTTON.md

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+# KeyButton
+
+Owner-approved momentary keyboard/pointer visualization (Mission Core, 2026-09-25).
+Outlined square key surface; pressed is white with black text in every theme.
+This explicit outline identifies a key, not a decorative panel border.
+
+`label` is the complete accessible action; children are the short key legend.
+`pressed` is controlled and never toggled by the component. Native button keyboard
+semantics, disabled behavior and focus indicator are retained. Consumers own
+pointer capture and release, keyboard scope, blur/cancel/unmount handling and
+any domain command lease. A displayed pressed key is not an acknowledgement
+from equipment. Disabled always renders released. No global listeners exist.
+
+Catalog includes idle, pressed, disabled and focusable surfaces. Use only for
+momentary key input, not ordinary actions (Button) or mode choice (SegmentedControl).