a11y: close the accessibility gaps found in research-12

[thorwhalen]

Follow-up to research-12 (docs/research/acture_research_12 -- Accessibility APIs and Command Dispatch.md, §3 and §4.1). acture's CommandRecord already matches the cross-platform accessibility action shape (label, description, key binding, enabled, invoke). What's missing is the projections and a few small gaps. Each item is small, and none needs a new CommandRecord field unless item 4 finds a named consumer.

• Palette ARIA test. acture-palette-react gets the APG combobox pattern from cmdk 1.1, but nothing asserts it. Add a test covering roles, aria-activedescendant and the accessible name, plus the focus hand-off when the view switches to the picker chain or the handoff form.
• Hidden vs disabled. The palette drops commands whose when is false. Menus conventionally show them as disabled. Decide per surface and record the rule in acture-palette-design. A menu projection should map when to aria-disabled / AXEnabled.
• Dispatch-result announcer. Route Result (ok or error.message) to a polite aria-live region after the palette closes (WCAG 4.1.3). It is a small consumer or middleware and covers every surface.
• Toggle / checked state (design only). Checkable commands (menuitemcheckbox, AXMenuItemMarkChar, UIA Toggle) have nowhere to report their state. Prefer a separate state projection keyed by command id over a record field. Needs a named consumer first.
• aria-keyshortcuts in projections. Emit it from keybinding (with $mod resolved to Control or Meta) on menu and button projections, where the shortcut is live.
• docs/hand-written-native-menu.md. Project the registry into an Electron/Tauri native menu (label, accelerator, enabled, checked, click → dispatch). This exposes an acture app's commands to VoiceOver, the macOS Help-menu search, Raycast and Paletro.
• WCAG 2.1.4. Optionally add a single "disable character-key shortcuts" switch to acture-hotkeys. Per-binding unbind already meets the criterion, as research-10 §3.7 notes.