feat: auto shape completion

[andrinwinzap]

Automatic shape completion for the brush pen

Adds opt-in, OneNote-style shape recognition to the brush pen. When enabled, holding
the pen still at the end of a stroke replaces the drawn ink with the clean geometric
shape it resembles.

fixes #120

Motivation

Drawing exact shapes freehand is difficult, and switching to the shaper pen for a
single rectangle interrupts the flow of writing. Letting the brush recognize shapes
on demand keeps the user in one tool. Recognition is deliberately not run on every
pen-up: doing so makes the brush unpredictable, since any stroke that happens to look
like a line or a loop would be silently rewritten. Requiring an explicit hold makes
the replacement something the user asks for rather than something that happens to them.

Changes

rnote-compose: new shaperecognition module.
recognize_shape(&PenPath) -> Option<Shape> resamples the stroke to equal arc-length
spacing, classifies it as open or closed from the ratio of the endpoint gap to the
perimeter, and then dispatches:

• open strokes are fitted against their chord and accepted as a Line
• closed strokes are scored for "cornerness" from the distribution of turn angles
  along the loop; low-corner strokes are fitted as an Ellipse through the second
  moments of the outline, and high-corner strokes have their corners extracted and
  are validated as a Polygon
• a four-corner polygon with near-right angles is emitted as a Rectangle,
  snapping to axis-alignment when it is within a few degrees

Recognized shapes are lines, circles, ellipses, rectangles (including rotated ones),
and closed polygons of up to six corners. Every fit is validated against the original
stroke by residual/deviation thresholds, so strokes that do not clearly resemble a
shape are left as ink. The module is dependency-free and covered by unit tests for
each shape class, plus rejection tests for scribbles and degenerate strokes.

rnote-engine: hold-to-trigger in the brush pen.
While drawing, the brush tracks an anchor position and restarts a one-off timeout task
whenever the pen leaves a small radius around it. When the pen rests for the hold
duration, the task emits a new EngineTask::BrushRecognizeShape, which attempts
recognition and swaps the in-progress stroke for the recognized shape while the pen is
still down. Subsequent motion is swallowed until pen-up, so the shape does not get
re-drawn over.

Two constants govern the interaction, both in brush.rs:

• HOLD_DURATION = 700ms — long enough not to fire during the natural pause at the
  end of a stroke, short enough not to feel like waiting.
• HOLD_RADIUS_SURFACE = 6.0 (surface coords) — tolerates hand tremor and pen
  jitter while resting, without swallowing slow deliberate movement.

Both are hardcoded rather than exposed in the UI, to avoid adding a preference for a
value most users will not tune. Happy to surface them if you would prefer.

rnote-ui: a "Shape Recognition" group in the brush page with a single
Recognize Shapes switch. The feature is off by default, so existing behaviour is
unchanged unless the user opts in. The setting is stored as
BrushConfig::shape_recognition_enabled; BrushConfig already carries
#[serde(default)], so configs written by older versions continue to load.

Recognized shapes are drawn with the brush's current colour and width. Textured strokes
fall back to a smooth style with a constant pressure curve, since shapes cannot be
composed with the textured style.

Screenshots / screen capture

Screencast_20260709_163543.webm

Testing

• cargo fmt --check is clean and cargo clippy --workspace --all-targets reports no
  warnings.
• cargo test --workspace passes, including 8 new unit tests in
  rnote-compose::shaperecognition covering line, circle, flat ellipse, axis-aligned
  rectangle, rotated rectangle and triangle recognition, plus rejection of scribbles
  and strokes below the minimum size.

AI disclosure

This contribution was developed with the assistance of LLM/GenAI tools.