feat: act on the app's own controls in-process (elementActionRequest)

[sarensw]

Why

Launch parameters put an app into a state. Some screenshots also need its UI driven — MacPacker's search field has to be focused for the search shot, which until now the app did itself on seeing -SearchQuery, i.e. screenshot-shaped behaviour living in shipping window code.

Driving it from outside would mean synthesizing input, which needs the Accessibility grant. Driving it inside the target does not: a process may read and drive its own accessibility tree freely. That is the same trick ElementLocator.screenFrame already uses for element frames — this just adds acting to looking.

What

elementActionRequest names a control by accessibility identifier and says what to do to it. ElementLocator.perform resolves it with the same in-process self-read and replies elementActionResult — whether it worked, and if not why, so a plan naming a control the app doesn't expose fails legibly instead of producing a screenshot of the wrong thing.

public struct ElementAction: Codable, Sendable {
    public enum Kind: String, Codable, Sendable { case focus }
    public let identifier: String
    public let kind: Kind
}

One kind, focus. Add another when a plan needs one.

Neither side learns anything about any app's UI: the identifier comes from whoever wrote the plan. The app's only obligation is ordinary accessibility — .accessibilityIdentifier(_:) on the control.

Verified

swift test: 10 tests in 3 suites, all passing — both new message shapes round-trip, and acting on an unknown identifier reports why rather than silently succeeding.

Consumers build clean against this branch (SandboxPilot and MacPacker).

Not yet verified end to end: that a real focus reaches a real control. This machine has no Accessibility/Automation grant for the harnesses that could check it independently — the XCUITest runner fails with "Timed out while enabling automation mode" — so it needs a run of MacPacker's 04-search scenario through a SandboxPilot built from this branch. Worth doing before tagging.

Needs a 1.4.0 tag after merge; LeanBytes/SandboxPilot#32 and sarensw/MacPacker#182 both pin it.