docs(ui-settings): describe the settings domain base and its shell

`SettingsRoot` and the `sidebar.settings` occupancy moved to ui-settings-general,
but both READMEs still read as if ui-settings were the shell. ui-settings now
documents what it actually is — the `ctx.settingsScope` transport plus the slot
type declarations, with the bind ownership, refresh triggers, write revision
handling, and decode refusal a preference row must know — and records the
loopback-only and single-field write gaps in place of two limitations that no
longer exist. ui-settings-general documents the shell it now renders. The stale
module JSDoc on both halves follows.

packages/client/ui-settings/src/index.ts

@@ -1,4 +1,4 @@
 /** Host loader entry for the browser implementation exported from `./client`. */
 
-/** Host plugin body — no host-side behavior for the settings shell plugin. */
+/** Host plugin body — no host-side behavior for the settings domain base plugin. */
 export function apply(): void {}