Add Polish (pl) language localization

[michaldziwisz]

This PR adds a Polish localization for MathCAT's speech and navigation rules.

What's included (all under Rules/Languages/pl/, 13 rule files):

• ClearSpeak_Rules.yaml, SimpleSpeak_Rules.yaml
• SharedRules/ (default, general, geometry, linear-algebra, calculus)
• definitions.yaml, unicode.yaml, unicode-full.yaml, intent.yaml, navigate.yaml, overview.yaml

Plus regression tests:

• tests/Languages/pl.rs -- Polish rule regression tests
• tests/languages.rs -- registers the pl test module

Notes:

• No changes to the engine core (src/ untouched).
• Written against the crate version currently on main (0.7.6-beta.5).
• The rules have been extensively tested against corpora generated via pandoc and against real-world MathML from Wikipedia (Mathoid). Continuous reading is correct across all tested domains; the remaining known navigation limitations for a handful of nested-norm (U+2225) and degree (degC) cases are language-independent core-navigation behaviors (see e.g. Navigation issue on <msubsup> #415), not specific to these rules.

Happy to adjust anything based on your review.