feat(math): exact evaluation of handwritten expressions (Issue #46, stage 1)

New module math.rs: solve() reads a recognised expression such as 2^4,
2,5 · 3 or (x + 3)² and returns its result as text (16, 7,5,
x² + 6x + 9). Own parser with exact fractions and polynomial expansion
instead of evalexpr, as implicit multiplication and expanding need it
anyway. Not used by the UI yet.

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

README.de.md

@@ -191,6 +191,7 @@ src/
 ├── storage.rs      # Laden/Speichern (atomar), Notizbücher, Papierkorb, Speicherort
 ├── disk_watch.rs   # Änderungen anderer Geräte übernehmen (Sync-Clients)
 ├── library.rs      # Ordner der Notizbücher (library.json)
+├── math.rs         # Exaktes Ausrechnen handgeschriebener Formeln (Issue #46)
 ├── autosave.rs     # Speichern im Hintergrund
 ├── export.rs       # PDF-Export
 ├── page_export.rs  # Seite als PNG/SVG, Auswahl als Bild