[KS][KEY] shortcut-map-rebind: ⌘⇧N fires nothing under the driver #450

Part of #15 · Found in #446

Problem

e2e/flows/keyboard/shortcut-map-rebind.yaml fails under --backend driver on origin/seth/kitchen-sink-gate (bc15fee, run 2026-10-13): after rebinding New note to ⌘⇧N, pressing ⌘⇧N fires nothing (keyboard-shortcut-map-fired stays "Press a binding to fire its command."). It fails the same way with and without the #446 change.

Likely cause, not confirmed in the app: on macOS 27, -[NSMenu performKeyEquivalent:] matches a ⌘⇧N key-down against an item whose key equivalent is plain ⌘N. A standalone AppKit probe (File ▸ New n+⌘ with newDocument:, then a Spark-style item n+⇧⌘, synthetic ⇧⌘ event with characters N/n and charactersIgnoringModifiers n) runs File ▸ New and returns YES. The Kitchen Sink template has File ▸ New ⌘N, so the main menu consumes ⌘⇧N before the shortcut map's menu item or router sees it.

Scope

• Confirm what consumes ⌘⇧N in the driven app (menu item, router, or the driver's synthesized event)
• Fix it, or make the flow use a binding that does not depend on it, and report the cause