i18n: the effect catalog takes a language's tables, with Japanese's slot reserved (#821)

The Japanese data point in #821 (zitann2020) offered to write its rows against whatever mechanism
this PR lands, so both languages share one catalog instead of inventing two. This makes adding a
language a data change and reserves `ja`:

- `Catalog` holds a language's effect rows and its category labels, and `catalog()` resolves a
  language code to one; adding a language is two tables and one arm, with no change to the lookups
  or to any panel.
- `JAPANESE_EFFECTS` and `JAPANESE_CATEGORIES` are declared, keyed by the same effect ids and by
  the registry's English category names, and `ja` resolves to them. Both are empty, so a Japanese
  interface draws exactly what it drew before — the slot changes no behaviour on its own.
- `a_pending_language_only_uses_known_ids` guards the pending tables (Japanese and Traditional
  Chinese) as rows arrive: an id Simplified Chinese does not know, a different English name for one
  of them, an empty translation or a repeated id fails. `languages_without_rows_draw_the_registry_name`
  asserts each pending language has an arm in `catalog()` (a table with no arm would leave a
  translator's rows dead) and is still row-less, so the day one gets rows the test says where its
  coverage moved.
- `only_the_brand_names_stay_in_english` now documents that it pins *Simplified Chinese's* five
  brand names and is deliberately not shared: AE's Japanese interface keeps the whole `CC`, `VR`
  and `OCIO` sets in English while its Chinese one translates them, so repeating the English name
  is a language's choice and not an unfinished row. The row check reflects that (a repeated English
  name is accepted, an empty one is not).

docs/localization-parity.md records the two empty tables and that each language decides for itself
which names stay in English.

Refs #821

docs/localization-parity.md

@@ -31,7 +31,12 @@ interface; Arabic and Hebrew text *in layers* is supported through its Middle Ea
   the drawing — the Effect menu, Effect Controls, the Timeline and Effects & Presets — goes
   through the catalog, so a project file is identical whichever interface language opens it.
   A repeat (`Gaussian Blur 2`) keeps the index the engine appends, and a group the user renamed
-  in Effect Controls stays verbatim. No other language has rows yet, so they draw English.
+  in Effect Controls stays verbatim. Adding a language there is two tables and one arm, and no
+  panel changes: Traditional Chinese and Japanese have their tables in place and empty, so those
+  languages draw English until a translator fills them (the Japanese set for #821 is what that
+  slot is for). Each language decides for itself which names stay English — Adobe's Japanese
+  interface leaves the `CC`, `VR` and `OCIO` sets in English while its Chinese one translates
+  them — so a row repeating the English name is a choice, not a missing row.
 - **Not translated anywhere yet:** effect *parameter* names (≈ 1,650, `crates/effects`),
   engine-generated text (property names, errors, undo labels), the Home screen's unwrapped
   literals (#574), Learn tutorials and documentation.