Add Mac native keyboard scanning and shared keyboard workflow

[enaboapps]

Goal

Integrate the native Mac Accessibility Keyboard with row-then-key switch scanning, preserving editor focus. Deliver the shared keyboard-scanning foundation first.

Behaviour

• Add Keyboard as the ninth main action-menu tile and optional press/hold action. Open or reuse the keyboard without clicking the selected point.
• Scan visible enabled keys by actual rows and positions; activate once and return to rows.
• Persistent Back to scanning and Close keyboard controls. Back leaves it open; Close hides it; both return to normal armed idle. Keyboard switch action while keyboard scanning performs Back.
• Reuse scan colour, timing, manual/automatic movement, reverse, pause and hold actions. Preserve emergency stop and prevent native keyboard output recursively activating switches.
• Refresh layout/geometry; reject stale activation. Errors retain a switch-accessible return.
• Shared typed actions, adapters and frontend states; existing settings remain valid, local/Remote parity, no BLE wire changes.

Qualification and validation

First prove native open/close, focus-preserving activation and generated-event identification in the Mac VM. Probe AX keys expose bounds but no press action: validate explicit-coordinate native clicks. Normalize glyph labels. Report blockers rather than substituting point scanning or ignoring all injected events.

Fake tests cover navigation, controls, timing, modifiers, repeated keys, stale layouts, cancellation, failures and exactly-once activation. VM tests use disposable text, mapped Space/Enter/letters, re-entry, focus, resizing and display changes. Restore settings, permissions, keyboard visibility and VM state. Never interrupt or replace the signed local Mac app.

Branch from current main, run required frontend/Rust checks, independent latest-head review and ready PR with passing CI. Merge/release require separate instructions.

[enaboapps]

Native qualification found a focus blocker, so the scanning feature is not implemented yet.

Mac VM: macOS 26.6.2 (25G83). Tests used a temporary Accessibility-authorized probe with its own disposable editor; the signed local Mac app was left untouched.

Confirmed:

• AX exposes the native keyboard's current key bounds. Coordinate activation typed repeated a, Space, Return and Delete into the disposable editor, with exactly one down/up pair per selection.
• Shift changes the discovered labels and a following letter types through the native keyboard.
• Native-generated key events in this VM carried source state 0 and user-data marker 9223372036854775806; the reported source PID was the target editor, not Assistive Control. A VM hardware Space event had source state 1, marker 0 and PID 0. A probe capture test passed native A/Space/Return while consuming physical Space. This is evidence for this OS build, not a documented universal origin contract or completed production filter.
• The native Close control hides the keyboard and returns virtualKeyboardOnOff to 0.

Opening gate failure:

• TIS enable/select/deselect calls return success but do not reliably show/hide the Accessibility Keyboard.
• Opening the Keyboard settings URL in the background and performing AXPress on AX_VIRTUAL_KEYBOARD works when System Settings is already running, and preserved the editor foreground PID in that case.
• On a cold System Settings start, the same background URL launch takes foreground focus. The editor PID was 1908 before opening; the post-enable and follow-up foreground PID was 2010, confirmed as System Settings. Typing must not proceed in that state.

A production adapter therefore still needs a qualified opening/focus-restoration transaction, including cancellation and intervening user focus changes. The current experiment is not suitable to ship as the Keyboard action. Full row/key navigation, stale-target tests, native layout/display testing and production generated-event handling remain outstanding.

Restoration completed: native keyboard off, temporary probe Accessibility approval revoked, original probe bundle restored byte-for-byte, Switchify in the Mac VM left stopped as found. Windows qualification is separately blocked by production signing branch policy (see #787). Both VMs are confirmed back in their original suspended states. Windows still has its original Switchify process (PID 9404) and OSK remained closed. The temporary transfer server was stopped.