i18n: add Polish (pl) interface language

Complete Polish catalog (4,492 rows): every menu string, tl! literal,
plural and status/error message, enforced by the existing completeness
tests (complete_menus, KEEPS_MENU_NAMES, COMPLETE_MESSAGES).

- pl.tsv: clean-room translation from the English meaning, following
  common Polish desktop-software conventions (obszar kompozycji for
  artboard, obszar roboczy for workspace, obrys, wypełnienie, próbki…)
- plural_polish: one / few (2–4, not 12–14) / many (0, 5–21, 25…)
- every pl-* locale (pl-PL, pl_PL.UTF-8) resolves to it
- tests: Polish plural forms, vocabulary, nested message translation;
  Polish letters in the bundled-font coverage test
- docs: README, development.md, localization-parity.md

Drafted with AI assistance; native review welcome.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DukHNvUY45rjRnvtz91x33

README.md

@@ -220,7 +220,7 @@ doesn't write Affinity files. Current `.af` validation includes 33 pinned files,
 and every-board SVG/PDF/PSD export, with fixes for Affinity 3 artboards, source-backed JPEGs and text runs.
 [Scope and limits](crates/affinity/README.md); [source audit and remaining gaps](docs/affinity-validation.md).
 The interface
-speaks English, German, Japanese, Korean, Vietnamese, Traditional and Simplified Chinese, Spanish, French, Hungarian, Italian, Russian and Ukrainian (and Czech and Brazilian Portuguese in the menus);
+speaks English, German, Japanese, Korean, Vietnamese, Traditional and Simplified Chinese, Spanish, French, Hungarian, Italian, Polish, Russian and Ukrainian (and Czech and Brazilian Portuguese in the menus);
 [localization-parity.md](docs/localization-parity.md) has the details.
 The scores are self-assessed from our code against Illustrator's public documentation;
 [target-app-parity.md](docs/target-app-parity.md#how-this-was-measured-2026-10-10) explains how far to trust them.