feat: freehand polygon drawing (press-and-drag) behind feature flag

[ozymandiashh]

Problem

The accepted community PRD in #8315 describes the current limitation:

Drawing polygons by clicking with the mouse for every point of the polygon is less optimal compared to simply being able to draw a polygon by holding down the mouse. Also the clicking technique does not work well with touchscreen devices or drawing pens, because the hovering feature is not easily available to these devices.

Solution

• Add a pointer-events drawing path for primary mouse, pen, and touch input.
• Simplify sampled points with a small in-house Ramer-Douglas-Peucker implementation.
• Convert coordinates at sample time while keeping simplification in zoom-independent screen space.
• Require a deliberate 5-pixel displacement before freehand semantics suppress compatibility mouse events.
• Reuse the stock canvas target checks so existing regions, anchors, transformers, and read-only annotations keep their normal interactions.
• Preserve the existing Cmd/Ctrl override for intentionally drawing over another region.
• Scope touch-action suppression to the selected Polygon tool.
• Commit each completed contour as one undoable history action.
• Release the history transaction safely when completion is cancelled by a tool switch or teardown.
• Guard the behavior with fflag_feat_front_polygon_freehand, default off.
• Preserve the existing click-to-place Polygon workflow with the flag both off and on.
• Add no runtime or development dependencies.

User Stories

• As an annotator, I want to draw polygons by simply dragging the mouse, so that I can annotate faster.
• As an annotator, I want to draw polygons via touchscreen or pen devices, so that I can annotate faster and more comfortably.

Acceptance Criteria

1. Given fflag_feat_front_polygon_freehand is disabled, when an annotator uses the Polygon tool, then the existing click-to-place workflow is unchanged and a pointer drag does not create a contour.
2. Given the flag is enabled and the Polygon tool is active, when an annotator presses, drags through at least three distinct points, and releases with a primary mouse pointer, then one simplified polygon is created.
3. Repeat criterion 2 with a pen, then with primary touch; each input creates exactly one polygon.
4. Given the flag is either enabled or disabled, click four vertices and close the polygon; exactly one stock click-to-place polygon is created.
5. Given a pointer moves less than 5 pixels before release, the gesture continues through the stock click path and creates no freehand contour.
6. If the stage transform changes during a drag, each committed vertex retains the coordinate transform active when that sample was collected.
7. After any completed freehand contour, one Undo action removes the whole contour and one Redo action restores it.
8. If the active pointer is cancelled, leaves the stage in a capture-less environment, the tool changes, or the component unmounts, then the in-progress trace is discarded without leaving drawing or history state stuck.
9. A freehand trace produces no additional network request and requires no new package.

Testing

• yarn lsf:unit --runInBand --silent: 129 test suites passed; 3,331 tests passed and 5 were skipped.
• yarn lsf:unit --runInBand --silent --testPathPatterns='components/ImageView/__tests__/ImageView.test.jsx|tools/__tests__/Polygon.test.js|utils/__tests__/freehand.test.js': 3 suites and 85 tests passed.
• BABEL_ENV=test MODE=standalone yarn nx run editor:build:production --skip-nx-cache: production build compiled successfully.
• Added Cypress coverage for flag-off drag behavior, click-to-place coexistence with the flag off and on, mouse, pen, touch, compatibility mouse events, and single-undo behavior.
• The integration spec passes locally on macOS with Cypress 14.5.0 and Electron against the production standalone build: all 7 tests passing.
• Regression coverage verifies that freehand does not start from existing regions, transformer anchors, or read-only annotations, while the Cmd/Ctrl override remains available.

Credit and context

Thank you to @cozeybozey for the PRD and proof of concept in #8315 and the earlier exploration in #8388. This implementation is intentionally scoped to the generic freehand Polygon workflow and uses pointer events plus an in-house simplifier without the extra dependency or unrelated changes from the prior attempt.

The interaction model has been production-tested with a medical imaging annotation team using tablets and styluses.

Closes-PRD: #8315

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[ozymandiashh]

Quick demo from real production use on dental panoramic radiographs. Apologies for my shaky hand while drawing the lines, but it shows how easily and quickly a contour can be repaired, and the precision you can get with press-and-drag compared to placing points one by one.