feat(gui): a ble_address schema field, so any BLE device can offer Scan

[gbradham]

Second of three for making Maimu installable as a plugin. Follows #135 (node extension points); the Maimu move itself is next.

The gap

A device rendered through the generic schema form got a plain text box for its address. Only the built-ins have a Scan button, because hardware_panel special-cases them by name — which a plugin device by definition cannot be.

Pasting a BLE MAC by hand is the difference between a device a researcher can add and one they have to be walked through. Moving Maimu to a plugin without this would have been a straight downgrade.

What this adds

A "ble_address" field type in schema_form, usable from any device's SETTINGS_SCHEMA:

{"key": "address", "label": "Address / UUID", "type": "ble_address"}

It also moves the widget itself out of hardware_panel. The hand-built dialog rows and the schema-rendered form a plugin gets are now the same widget rather than two that drift apart. That is the third duplicated-implementation bug this week (#129 two file formats, #133 two hardware-population loops), so writing a second copy here would have been a poor joke.

Degradation is deliberate

Scanning is async and the form has no event loop of its own, so the caller passes a runner. Without one the field still renders and stays editable — only the button is dropped. An address can always be typed, and a form that refused to render because it could not scan would be worse than one that simply cannot scan.

Reading it back

All three ways an address arrives are handled: picked from a scan, where the address is item data behind an advertised-name label; typed by hand; or pasted as a whole scan label, where a trailing " (name)" is stripped.

Tests — 8 new

Scan offered when a runner is supplied; degrades to a typeable field without one; clicking Scan actually runs the coroutine; a typed address reads back; a scanned entry reads back its address, not its label; a pasted label is stripped; a saved address is shown; an empty field reads back empty.

Verification

PYTHONPATH=src QT_QPA_PLATFORM=offscreen pytest tests/ — 3680 passed, 4 skipped, ruff and black clean.