Switch language in place, without a reload

Choosing a language in the picker now applies it at once. The choice is
stored and ?lang is kept in step through replaceState, and the picker keeps
focus.

Code that writes translated text itself re-presents it on a switch:
onLocaleChange() lets an owner listen, removed at teardown, and status
writers hand over a function of the locale (voice cards, scene director,
SDR, draw hints, imagery split, map sources, Bhote Koshi media) so the
words are rebuilt rather than re-sent. The HUD, cockpit, radio, CCTV,
launch, awareness and credit texts follow the same hook, and
applyDocumentTranslations can be applied again without undoing text code
wrote since.

The unsaved-work registry and persistLocaleAndReload existed only to guard
the reload, so they go, with their catalog entries.

docs/I18N.md

@@ -47,7 +47,17 @@ Static page markup that no view renders (the loading screen in `index.html`, a h
 <input placeholder="Search any location..." data-i18n-placeholder />
 ```
 
-`data-i18n` replaces the element's text, so put it on the element that directly holds the text, never on a parent with child elements. `data-i18n-title`, `data-i18n-aria-label` and `data-i18n-placeholder` translate that attribute. `data-i18n-context="radio tag"` gives the element's text a context, the catalog entry `radio tag|All`, as `t('All', { context: 'radio tag' })` does in code. Attribute markup is translated once at boot. Switching language still reloads the page: views would follow a new locale, but much of their text is published already translated by code that does not. A module that holds work a reload would lose (an unsaved edit, an open session) registers it with `registerUnsavedWork(() => busy ? msg('unsaved …') : null)` from `src/i18n/index.js`, and the language picker asks before reloading.
+`data-i18n` replaces the element's text, so put it on the element that directly holds the text, never on a parent with child elements. `data-i18n-title`, `data-i18n-aria-label` and `data-i18n-placeholder` translate that attribute. `data-i18n-context="radio tag"` gives the element's text a context, the catalog entry `radio tag|All`, as `t('All', { context: 'radio tag' })` does in code. Markup is translated at boot and again after every language switch.
+
+## Switching language
+
+The language picker switches in place, without a reload. A view that calls `t()` while it renders follows by itself. Code that translates text and hands it on (to a store, a published view object, a layer's state, a DOM node) must word it again when the language changes:
+
+- **Present again.** `onLocaleChange(listener)` from `src/i18n/index.js` calls the listener after each change and returns the function that removes it; call that at teardown. The listener presents the view again from what it keeps, as `layerPanel.js` and `cockpitController.js` do.
+- **Keep the English, translate when presenting.** Store `msg()` text or a function that words it, not its translation: the voice control takes `update(part, { text: () => t('MIC') })`, the scene director's status and the SDR receiver's messages keep such functions, and `imagerySplit.js` takes its titles that way.
+- **Translated constants in a view** go in `computed`, not in a plain `const` set up once.
+
+A toast already on screen keeps its words.
 
 ## Rules