feat(helper-lines): custom snapping guides + edge snapping for text & paint (#833, #834)

[hm21]

Implements #833 and #834.

#834 — App-defined custom snapping guide lines

Adds an API for apps to define their own snapping/helper guides (safe areas, thirds, columns, layout slots, …):

ProImageEditorConfigs(
  helperLines: HelperLineConfigs(
    customGuides: [
      HelperGuideLine(
        axis: Axis.vertical,
        position: 1 / 3,
        positionMode: HelperGuidePositionMode.normalized,
      ),
      HelperGuideLine(axis: Axis.horizontal, position: 120), // absolute px
    ],
    style: HelperLineStyle(customGuideColor: Color(0xFF00BFA5)),
  ),
)

• HelperGuideLine — axis (vertical snaps X, horizontal snaps Y), position, and positionMode (absolute editor-body pixels or normalized 0.0–1.0).
• Custom guides participate in layer snapping (reusing releaseThreshold) and are drawn with the new HelperLineStyle.customGuideColor while a layer snaps to them — even when showLayerAlignLine is false.
• Resolved against the live editor-body size, so snapping stays consistent across resizes (the snap always lands on the drawn line).
• New Custom-Guide-Lines example (thirds + safe-area guides).

#833 — Text & paint snap by their edges

Text and paint layers now expose their left / center / right and top / center / bottom edges as snap anchors (no dependence on TextAlign). When aligning to another layer or a custom guide, whichever edge sits closest snaps to it, making it easy to align text/paint blocks by their visible edge. Other layer types keep their single configured anchor; the editor center line still snaps by the layer center.

Implementation notes

• LayerInteractionManager._updateAlignmentGuides was generalized to multiple active-layer anchors (text/paint edges, both axes) and multiple targets (other layers' anchors + custom guides), keeping the existing snap/release hysteresis. A isVerticalGuideCustom/isHorizontalGuideCustom flag selects the draw color.
• Layer.renderSize exposes the rendered (already-scaled) content size used to derive edge anchors; calculateMovement receives editorBodySize to resolve custom-guide positions into the editor's center-relative coordinate space.

Tests

Added unit tests for HelperGuideLine.resolvePosition (absolute/normalized) and equality. Full suite (436 tests) passes and dart analyze is clean for both the package and the example.