feat(math): recognize handwritten expressions (Issue #46, stage 2)

New module math_ink.rs turns the strokes of an expression into text for
math::solve. Consecutive overlapping strokes are grouped into symbols,
each symbol is classified with the $P point-cloud recognizer (plus the
turning angle and a small stroke-count penalty) against built-in
templates for digits, + ( ) / x y a b, and the structure comes from the
positions: stacked lines are =, stacked dots :, lines with symbols above
and below are fraction bars, low dots between digits decimal commas and
raised symbols exponents. Own templates can be added for stage 4.

Tested with synthetic handwriting (slanted, rotated, warped); not used by
the UI yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -194,6 +194,7 @@ src/
 ├── disk_watch.rs   # Taking over changes from other devices (sync clients)
 ├── library.rs      # Notebook folders (library.json)
 ├── math.rs         # Exact evaluation of handwritten math (Issue #46)
+├── math_ink.rs     # Recognition of handwritten math ($P point clouds)
 ├── autosave.rs     # Saving in the background
 ├── export.rs       # PDF export
 ├── page_export.rs  # Page as PNG/SVG, selection as image