i18n: Dutch translates status and error messages

Dutch was not in COMPLETE_MESSAGES, so with the interface in Dutch
script errors, plug-in manifest and permission errors, parameter
schema errors, 3D effect refusals, file-reader errors and data-set
messages showed in English.

- nl.tsv gains 557 `@msg` rows and one `@msgplural` row: the messages
  the German catalog translates and Dutch lacked. Most are one block
  with a header comment on their conventions, in the order de.tsv
  holds them; the rest sit next to the related existing rows.
  Backticked names, command ids, double-quoted values and bare true,
  false and null stay as the English writes them.
- nl joins COMPLETE_MESSAGES, so the message tests that iterate the
  list check Dutch too, and the untranslated report lists Dutch's
  missing messages. It lists the same set for Dutch as for German.
- The registry comment, docs/development.md, the Dutch row and
  introduction of docs/localization-parity.md, and the interface
  languages line of docs/target-app-parity.md list Dutch among the
  languages that translate messages.

docs/development.md

@@ -294,7 +294,7 @@ other `sv-*` locale resolve to it), Georgian (`ka`, complete; `ka-GE` and `ka_GE
   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, Greek, Spanish, French, Georgian, Hungarian, Italian, Japanese, Korean, Polish, Brazilian Portuguese, Russian, Simplified Chinese, Traditional Chinese, Swedish, Ukrainian, Vietnamese, Finnish, Turkish and Catalan
+  placeholders are translated in turn (the reason after `: {e}` is often a message too). Czech, German, Greek, Spanish, French, Georgian, Hungarian, Italian, Japanese, Korean, Polish, Brazilian Portuguese, Russian, Simplified Chinese, Traditional Chinese, Swedish, Ukrainian, Vietnamese, Finnish, Turkish, Catalan and Dutch
   translate status and error messages (`COMPLETE_MESSAGES` in `i18n::tests`); the untranslated report lists the
   message literals the scan finds that one of them has no row for. The scan reads `Err("…")`, `Other(…)`,
   `#[error(…)]` and `status(…)` messages, messages given to an error enum's variant (`PsError::Limit(…)`, the reason

docs/localization-parity.md

@@ -15,7 +15,7 @@ translation never fails a test or a build: the interface shows the English text
 `cargo xtask ci` prints the untranslated report (`cargo xtask i18n-report` alone), which lists per language the
 strings missing from each catalog with the `complete_menus` flag and the messages missing from each language in
 the `COMPLETE_MESSAGES` list. Every non-English language marked "full" has the flag, and all of them except
-Bulgarian, Danish, Indonesian and Dutch are in the list. No language has help content: there is no translated (or
+Bulgarian, Danish and Indonesian are in the list. No language has help content: there is no translated (or
 English) in-app help, and the docs are English.
 
 **Native review:** every catalog was written clean-room from the English meaning, most of it by agents
@@ -51,7 +51,7 @@ plus review. Native-speaker review is human work and isn't in the hours.
 |---|---|---|---|
 | Traditional Chinese (Taiwan) | `zh-hant` | 4,892 rows, 790 of them `@msg` (counted 2026-10-21): menus, `tl!` literals and messages complete when counted (80% overall) | partial |
 | Italian | `it` | 4,011 / 4,020 | full |
-| Dutch | `nl` | 5,317 rows, 777 of them `@msg` (counted 2026-10-21): menus and `tl!` literals complete when counted; its messages are partial; one/other plurals | full |
+| Dutch | `nl` | 7,299 rows, 1,609 of them `@msg` and 47 `@msgplural` (counted 2026-11-13 on `main` at `9a1a158a3`): in `COMPLETE_MESSAGES`, and the untranslated report lists the same missing messages for it as for German; one/other plurals | full |
 | Danish | `da` | 5,279 rows, 750 of them `@msg` (counted 2026-10-21): menus and `tl!` literals complete when counted; its messages are partial; one/other plurals | full |
 | Russian | `ru` | 4,009 / 4,020 | full |
 | Ukrainian | `uk` | 4,009 / 4,020 (one/few/many plurals) | full |