fix: picker first-mouse acceptance + event-based drag coords; in-process picker selftest

Root cause: RegionPickerView lacked acceptsFirstMouse — as an LSUIElement accessory app
Shotdeck is never active when the hotkey fires, so the user's first click on the overlay
was refused and the drag never started. Also plumbs the monitored event's location through
the controller (hardware-cursor reads made the chain untestable). Adds PickerSelfTest
(SHOTDECK_PICKER_SELFTEST): posts synthetic mouse events through the app's own queue,
asserts the exact CaptureRegion, saves a mid-drag overlay bitmap. Coordinator ran it:
PICKER-SELFTEST PASS rect=(200.0, 729.0, 400.0, 300.0); overlay bitmap shows dim+punch+chip.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Sources/Shotdeck/RegionPickerWindow.swift

@@ -29,6 +29,7 @@ final class RegionPickerWindow: NSPanel {
         becomesKeyOnlyIfNeeded = false
         animationBehavior = .none
         collectionBehavior = [.canJoinAllSpaces, .fullScreenAuxiliary, .stationary]
+        acceptsMouseMovedEvents = true
         contentView = pickerView
     }
 
@@ -53,6 +54,8 @@ private final class RegionPickerView: NSView {
 
     override var isOpaque: Bool { false }
     override var acceptsFirstResponder: Bool { true }
+    // Accessory-app trap: first click on an inactive overlay is first-mouse and is dropped unless accepted.
+    override func acceptsFirstMouse(for event: NSEvent?) -> Bool { true }
 
     override func draw(_ dirtyRect: NSRect) {
         super.draw(dirtyRect)