i18n: Italian interface (it)

Adds an Italian catalog (crates/ui-egui/src/i18n/it.tsv, 3,549 rows in
es.tsv's order) covering every menu string, every tl! literal, the
plurals and every status/error message, at the same scope as Spanish.
Standard Italian in sentence case, written clean-room from the meaning
of the English labels; the product, workspace, perspective preset and
language names stay in English as in es and ja.

Registered as "Italiano" with the one/other plural rule and
complete_menus; every it-* locale resolves to it. Tests: it joins
KEEPS_MENU_NAMES and COMPLETE_MESSAGES, italian_reads_as_italian checks
vocabulary and plurals, tags_map_to_languages the locale tags, and the
bundled-font glyph test covers Italian accents and guillemets. README,
ROADMAP and docs/development.md list the language.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_011iayep93edeimiAE8oSfaT

docs/development.md

@@ -124,7 +124,8 @@ Languages shipped: English (`en`, the source), Traditional Chinese (`zh-hant`, c
 in Taiwan; `zh-TW`, `zh-HK`, `zh-MO` and `zh-Hant-*` locales all resolve to it), Simplified Chinese (`zh-hans`,
 complete, in the vocabulary used in mainland China; `zh-CN`, `zh-SG`, `zh-Hans-*` and a bare `zh` resolve to it,
 so the two scripts never mix), Japanese (`ja`, complete), Spanish (`es`, complete, in neutral
-international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or `es-419` resolves to it), Czech (`cs`, every menu label) and Brazilian Portuguese
+international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or `es-419` resolves to it), Italian (`it`,
+complete; `it-IT`, `it-CH` and every other `it-*` locale resolve to it), 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
@@ -152,10 +153,10 @@ international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or
   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). Spanish covers every
+  placeholders are translated in turn (the reason after `: {e}` is often a message too). Spanish and Italian 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 an `es.tsv`
-  row (`VECTORCRAFT_I18N_DUMP_MESSAGES=messages.txt cargo test -p vectorcraft-ui-egui
+  and an `it.tsv` row (`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
   until they add `@msg` rows.
 - Not translated on purpose: names that are user data (layers, swatches, fonts, documents), the tab