doc(web): own the Web/TUI split on an unlisted current model

The TUI still renders the unlisted current model as its own row and marks it
current; Web shows the unset trigger label and asks for a replacement. The
note recorded the Host decision but not that the two surfaces present it
differently, which reads as a missed migration rather than a choice.

Also records why `trigger.selectAria` and `trigger.fallback` hold identical
strings, so a future coalescing cleanup does not merge them.

packages/client/ui-model/src/client/locales.ts

@@ -1,4 +1,12 @@
-/** `model` namespace dictionaries. */
+/**
+ * `model` namespace dictionaries.
+ *
+ * `trigger.selectAria` reads identically to `trigger.fallback` today and is
+ * still a separate key: the visible fallback label and the accessible name of
+ * an unset trigger are free to diverge per locale, and folding it into
+ * `trigger.aria` would announce the degenerate "Select model, current Select
+ * model".
+ */
 
 /** Simplified Chinese dictionary (the key-set source of truth). */
 export const zh = {