fix: the locale runtime the card ships is the code the tests prove (#354)

The production LANGUAGE_RUNTIME was a handwritten twin of the tested
LanguageRuntime class (germanDictionary/Pending/Failed): equivalent on the
day it was written, invisible to every i18n-runtime test afterwards. The
registry now exports one page-scoped `new LanguageRuntime(LANGUAGE_REGISTRY,
…)` instance — the whole existing suite starts proving the object production
actually runs, and a contract unit (instanceof + source free of the old
field names) keeps the duplicate from returning.

The class gains an optional `loadFailed(code)` hook — fired once when a
dictionary load settles into English fallback — and the registry fans it out
through `subscribeLanguageLoadFailures`. Only the View card subscribes (it
alone owns toast infrastructure): a failed language pack now shows the new
`toast.locale_load_failed` message (en/ru/de) instead of a console-only
warning; space card and both GUI editors keep the console warning as before.

Proofs: contract unit, hook unit, subscription unit; smoke_german_locale
extended — the both-attempts-failed scenario now asserts the visible toast;
two new registry mutants (handwritten-twin returns, toast dropped).

Issue: #354
User-Visible: yes

src/houseplan-card.ts

@@ -232,7 +232,7 @@ import {
 } from './coordinate-canonicalization';
 import { enqueueSerializedWrite } from './serialized-write-queue';
 import { hasTranslation, langOf, t, type I18nKey } from './i18n';
-import { LANGUAGE_RUNTIME } from './i18n/registry';
+import { LANGUAGE_RUNTIME, subscribeLanguageLoadFailures } from './i18n/registry';
 import { languageLoadingTemplate, languageRenderGate } from './i18n/language-runtime';
 import { CommandStack } from './command-stack';
 import { resolvedSvgScreenBlend, svgScreenBlendSupported } from './glow-blend';
@@ -2189,6 +2189,7 @@ export class HouseplanCard extends LitElement {
   private _continuity = this._newContinuityController();
   private _continuityHistory: import('./visual-continuity').ContinuityTraceEvent[] = [];
   private _continuityUnsub?: () => void;
+  private _languageFailureUnsub?: () => void;
   private _continuityEpoch = 0;
   private _continuityDataReady = true;
   private _continuityPaintToken = -1;
@@ -2474,6 +2475,13 @@ export class HouseplanCard extends LitElement {
     if (resolvedBlend !== undefined) this._glowScreenBlend = resolvedBlend;
     this._continuityUnsub?.();
     this._continuityUnsub = subscribePageVisibility(this.ownerDocument, this._pageVisibility);
+    // #354: only the View card owns toast infrastructure, so it alone turns a
+    // locale-load failure into a visible message; other runtime surfaces keep
+    // the console warning from the shared LanguageRuntime.
+    this._languageFailureUnsub?.();
+    this._languageFailureUnsub = subscribeLanguageLoadFailures(() => {
+      this._showToast(this._t('toast.locale_load_failed'));
+    });
     super.connectedCallback();
     this._pointerModality.connect(this.ownerDocument.defaultView);
     const PointerHoverObserver = this.ownerDocument.defaultView?.MutationObserver;
@@ -2549,6 +2557,8 @@ export class HouseplanCard extends LitElement {
     window.removeEventListener('popstate', this._onLocationChanged);
     this._continuityUnsub?.();
     this._continuityUnsub = undefined;
+    this._languageFailureUnsub?.();
+    this._languageFailureUnsub = undefined;
     this._motionMedia?.removeEventListener?.('change', this._onMotionChange);
     this._motionMedia = undefined;
     if (this._vacRaf) { cancelAnimationFrame(this._vacRaf); this._vacRaf = 0; }

src/i18n/de.json

@@ -622,6 +622,7 @@
   "room.label_scale": "Metrikgröße",
   "preview.room_name": "Wohnzimmer",
   "toast.cfg_reload_failed": "Der Plan konnte nicht vom Server neu geladen werden: {err}",
+  "toast.locale_load_failed": "Das Sprachpaket konnte nicht geladen werden. Bis zum Neuladen der Seite wird Englisch verwendet.",
   "room.settings_short": "Raumeinstellungen",
   "room.unnamed": "Unbenannter Raum",
   "marker.use_climate_temp": "Geräte-Temperatur im Raum berücksichtigen",

src/i18n/en.json

@@ -622,6 +622,7 @@
   "room.label_scale": "Metrics size",
   "preview.room_name": "Living room",
   "toast.cfg_reload_failed": "Could not reload the plan from the server: {err}",
+  "toast.locale_load_failed": "Could not load the language pack. English is used until the page reloads.",
   "room.settings_short": "Room settings",
   "room.unnamed": "Unnamed room",
   "marker.use_climate_temp": "Include the device temperature in the room",

src/i18n/language-runtime.ts

@@ -37,6 +37,8 @@ export class LanguageRuntime {
     private readonly entries: readonly RuntimeLanguageEntry[],
     private readonly expectedFingerprint: string,
     private readonly warn: (message: string, error: unknown) => void = console.warn,
+    /** Optional #354 hook: a dictionary load settled into English fallback. */
+    private readonly loadFailed?: (code: string) => void,
   ) {
     for (const entry of entries) {
       if (entry.dictionary) this.dictionaries.set(entry.code, entry.dictionary);
@@ -84,6 +86,7 @@ export class LanguageRuntime {
     }
     this.failed.add(entry.code);
     this.warn(`[houseplan] unable to load ${entry.code} locale; using English`, lastError);
+    this.loadFailed?.(entry.code);
   }
 }
 

src/i18n/registry.ts

@@ -1,9 +1,10 @@
 import en from './en.json' with { type: 'json' };
 import ru from './ru.json' with { type: 'json' };
-import type {
-  LanguageRuntimeContract,
-  LazyLanguageModule,
-  LocaleDictionary,
+import {
+  LanguageRuntime,
+  type LanguageRuntimeContract,
+  type LazyLanguageModule,
+  type LocaleDictionary,
 } from './language-runtime';
 
 const BUILD_FINGERPRINT = '__HOUSEPLAN_SOURCE_FINGERPRINT__';
@@ -40,37 +41,35 @@ export type Lang = (typeof LANGUAGE_REGISTRY)[number]['code'];
 export const FALLBACK_LANGUAGE_CODE: Lang = 'en';
 export const FALLBACK_DICTIONARY = en;
 
-/** One page-scoped cache: multiple cards share the same locale request. */
-let germanDictionary: LocaleDictionary | undefined;
-let germanPending: Promise<void> | undefined;
-let germanFailed = false;
+/**
+ * #354: locale-load failures surface once through this page-scoped listener
+ * list. Only the View card subscribes and toasts; every other runtime surface
+ * (space card, both GUI editors) stays with the console warning below.
+ */
+const languageLoadFailureListeners = new Set<(code: string) => void>();
 
-async function settleGerman(): Promise<void> {
-  let lastError: unknown;
-  for (const attempt of [0, 1] as const) {
-    try {
-      const loaded = await loadGerman(attempt);
-      if (loaded.fingerprint !== BUILD_FINGERPRINT) throw new Error('locale fingerprint mismatch');
-      germanDictionary = loaded.dictionary;
-      return;
-    } catch (error) {
-      lastError = error;
-    }
-  }
-  germanFailed = true;
-  console.warn('[houseplan] unable to load de locale; using English', lastError);
+export function subscribeLanguageLoadFailures(
+  listener: (code: string) => void,
+): () => void {
+  languageLoadFailureListeners.add(listener);
+  return () => languageLoadFailureListeners.delete(listener);
 }
 
-export const LANGUAGE_RUNTIME: LanguageRuntimeContract = {
-  state: (code) => code !== 'de' || germanDictionary
-    ? 'ready' : germanFailed ? 'fallback' : 'pending',
-  dictionary: (code) => code === 'de'
-    ? germanDictionary : code === 'ru' ? ru : code === 'en' ? en : undefined,
-  ensure: (code) => {
-    if (code !== 'de' || germanDictionary || germanFailed) return Promise.resolve();
-    return germanPending ??= settleGerman().finally(() => { germanPending = undefined; });
+/**
+ * The production runtime IS the tested class (#354): the previous handwritten
+ * object duplicated its logic, so the whole i18n-runtime test suite proved
+ * properties of code the card never ran. One page-scoped instance is shared
+ * by every card and editor; the warn hook keeps the console line and fans the
+ * failure out to subscribers.
+ */
+export const LANGUAGE_RUNTIME: LanguageRuntimeContract = new LanguageRuntime(
+  LANGUAGE_REGISTRY,
+  BUILD_FINGERPRINT,
+  console.warn,
+  (code) => {
+    for (const listener of languageLoadFailureListeners) listener(code);
   },
-};
+);
 
 /** Return a loaded dictionary, falling back synchronously to English. */
 export function dictionaryFor(value: unknown): LocaleDictionary {

src/i18n/ru.json

@@ -622,6 +622,7 @@
   "room.label_scale": "Размер подписей",
   "preview.room_name": "Гостиная",
   "toast.cfg_reload_failed": "Не удалось перечитать план с сервера: {err}",
+  "toast.locale_load_failed": "Не удалось загрузить языковой пакет. До перезагрузки страницы используется английский.",
   "room.settings_short": "Настройки комнаты",
   "room.unnamed": "Комната без имени",
   "marker.use_climate_temp": "Учитывать температуру устройства в комнате",