feat(editor): add DocumentDetector with order4Points and overlay setCorners

- DocumentDetector.kt: OpenCV-based document quad detection; order4Points
  pure function sorts 4 corners to TL/TR/BR/BL (sum/diff heuristic).
- DocumentDetectorTest.kt: TDD red→green cycle on order4Points; detect()
  uses OpenCV native and is not unit-tested.
- CornerOverlayView.kt: public setCorners(FloatArray) sets corners from
  external detection result, marks initialized, triggers invalidate().

Constraint: detect() requires OpenCV native init; only order4Points is
  unit-testable in JVM test scope.
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: wire DocumentDetector.detect() call from CameraActivity in next task
Tested: DocumentDetectorTest.orders4PointsToTlTrBrBl passes; assembleDebug succeeds
Not-tested: detect() on real Bitmap (needs instrumented test or device)
Related: feat/auto-edge-detection A-stage Task 2

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