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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01DukHNvUY45rjRnvtz91x33

docs/development.md

@@ -178,7 +178,8 @@ complete; `fr-FR`, `fr-BE`, `fr-CA`, `fr-CH` and every other `fr-*` locale resol
 complete; `it-IT`, `it-CH` and every other `it-*` locale resolve to it), Russian (`ru`, complete; every `ru-*`
 locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), Ukrainian (`uk`, complete; `uk-UA`,
 `uk_UA.UTF-8` and every other `uk-*` locale resolve to it; integer plurals use one/few/many), Hungarian (`hu`, complete;
-`hu-HU` and every other `hu-*` locale resolve to it; a count takes the singular, so plurals have one form), Czech (`cs`, every menu label) and Brazilian Portuguese
+`hu-HU` and every other `hu-*` locale resolve to it; a count takes the singular, so plurals have one form), Polish (`pl`, complete; `pl-PL` and every other `pl-*`
+locale resolve to it; plurals use one/few/many, with 21, 31 … taking many), Czech (`cs`, every menu label) and Brazilian Portuguese
 (`pt-br`, every menu label and every `tl!` literal). Untranslated text falls back to English until its rows are added.
 
 - `tl!("…")` translates a literal into the language the UI is drawn in; `i18n::t(s)` is the same for a
@@ -206,7 +207,7 @@ locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), Ukrainian (`uk`, com
   control channel, MCP and tests read them) and are translated only where the status bar draws them
   (`i18n::msg`). `@msg` catalog rows hold a whole message or a template such as
   `Couldn't open {name}: {e}`; `{_1}`, `{_2}` … stand for the format string's `{}`, and the values in the
-  placeholders are translated in turn (the reason after `: {e}` is often a message too). Czech, German, Spanish, French, Hungarian, Italian, Japanese, Korean, Russian, Ukrainian and Vietnamese cover every
+  placeholders are translated in turn (the reason after `: {e}` is often a message too). Czech, German, Spanish, French, Hungarian, Italian, Japanese, Korean, Polish, Russian, Ukrainian and Vietnamese cover every
   message literal the test scan finds (`complete_languages_translate_every_message`, languages listed in
   `COMPLETE_MESSAGES`): a new `Err("…")`, `Other(…)`, `#[error(…)]` or `status(…)` message needs a row in each of those catalogs (`VECTORCRAFT_I18N_DUMP_MESSAGES=messages.txt cargo test -p vectorcraft-ui-egui
   complete_languages_translate_every_message` lists them all). Other languages show messages in English