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>

src/lib.rs

@@ -18,6 +18,7 @@ pub mod i18n;
 pub mod image_layer;
 pub mod import;
 pub mod library;
+pub mod math;
 pub mod osk;
 pub mod page_export;
 pub mod paths;