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/main.swift

@@ -7,6 +7,9 @@ import ShotdeckCore
 if ProcessInfo.processInfo.environment["SHOTDECK_SNAPSHOT_DIR"] != nil {
     MainActor.assumeIsolated { PanelSnapshot.runIfRequested() }
 }
+if ProcessInfo.processInfo.environment["SHOTDECK_PICKER_SELFTEST"] != nil {
+    MainActor.assumeIsolated { PickerSelfTest.runIfRequested() }
+}
 ShotdeckApp.main()
 
 struct ShotdeckApp: App {