feat(editor): 8-point curve crop + CamScanner-grade pro filters v1.089

User compared our magic-color output against CamScanner's Magic Pro and
asked for parity, plus CamScanner's 8-point (curved-edge) crop.

8-point curved crop:
- CornerOverlayView gains 4 edge-midpoint handles (white, smaller) on top
  of the 4 corner handles. Each edge draws as a quadratic Bezier through
  its midpoint, so a curled paper edge can be matched by hand. Dragging a
  corner carries its two adjacent midpoints by half the delta, keeping a
  straight edge straight and a curved edge's bend intact.
- CurveWarp (new) flattens the 8-point curved boundary with a Coons patch
  (each edge a quadratic Bezier; coarse 80x80 grid -> OpenCV resize+remap,
  same strategy as UVDoc). EditorFlatten uses it only when an edge is
  actually bent (isCurved past 0.5% of the short side); otherwise the
  existing perspective warp runs, so the straight-edge case is unchanged.
  Math validated in the python spike: straight edges map corners exactly,
  a bent midpoint lies on the boundary, remap leaves 0% black.

CamScanner-grade filters (new ProFilters):
- Magic Pro: magicColor (illumination flatten) -> per-channel white-point
  normalize (65th pct -> 242) -> bilateral texture suppression -> levels
  (black 35 / white 205) -> sharpen. This pushes the background to true
  white and kills paper-wrinkle texture, matching CamScanner's Magic Pro
  (our old magic left a gray cast). Validated on the reported document.
- Deshadow: LAB L-channel background division (shadows gone, color kept).
- Eco: scan-gray + raised white point (ink-saving light scan).
- Editor and camera filter menus now expose 매직프로/음영제거/에코; the
  redundant enhance button was dropped (the function stays as a fallback).

Constraint: CurveWarp/PerspectiveTransformer never recycle their source; EditorFlatten owns intermediates and the caller owns hi. Curve warp only triggers when a midpoint is bent past threshold.
Confidence: medium-high
Scope-risk: module
Reversibility: moderate
Directive: on-device, confirm the 8-point handles drag smoothly under zoom and Magic Pro background is pure white; tune levels white point (205) if some papers stay gray.
Tested: assembleDebug + testDebugUnitTest exit=0 (25 tests, CurveWarpTest 3 new); python validation of Coons math, remap integrity, and Magic Pro background whiteness; manual memory/touch-path audit (no dangling enhance refs, recycle chain leak-free).
Not-tested: on-device 8-point gesture feel and filter look across diverse papers.
Related: CamScanner libpagescanner (CurveCropDewarp / 8-point), ENGINEERING_LOG section 4 (filters)

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