fix(editor): 8-point handles slide edges straight, not curved v1.090

User clarified the 8-point crop: the edge midpoints must move the whole
edge as a straight line (horizontal/vertical), not bend it into a curve.

- CornerOverlayView: edges are always straight lines again. The 4 edge
  midpoints are derived (edge centers), not stored curve state. Dragging
  a midpoint translates that edge along its own normal only (projects the
  finger movement onto the perpendicular), so both its corners shift by
  the same amount and the edge stays straight and parallel — a level top
  edge slides vertically, a side edge slides horizontally. Corner drag is
  back to moving just that corner.
- Removed CurveWarp (Coons patch) and its test; EditorFlatten and the
  editor no longer pass/route midpoints. Warp is always the 4-corner
  perspective transform; curl/crease flattening stays with UVDoc, which
  already handles it.

Constraint: edges stay straight; midpoint drag is perpendicular-only translation of both endpoints.
Confidence: high
Scope-risk: module
Reversibility: trivial
Directive: on-device, confirm the white edge handles slide each side in/out cleanly under zoom.
Tested: assembleDebug + testDebugUnitTest exit=0 (22 tests); edge-translation math checked (top edge -> vertical, side edge -> horizontal); no dangling CurveWarp/bmpMids refs.
Not-tested: on-device gesture feel.
Related: 33f2c99 (introduced the curved version now reverted)

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