fix(ui-slash): make the reference lexicon reactive end to end

The decoration scan read a mutable lexicon() aggregation during render
with no subscription, so a catalog settling or a child spawning after
prewarm left drafted tokens undecorated until an unrelated re-render.
The controller now publishes the aggregation as a snapshot store fed by
a new optional SlashSource.subscribeLexicon hook (ui-skill notifies on
settle/invalidate, ui-subagent forwards the session-list feed), the
composer keyboard face exposes it as an observable, and InputBar
subscribes through uSES. Sources registered after scope birth now warm
and join live controllers via a service broadcast.

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -56,7 +56,11 @@ function bench(over?: BenchOptions) {
     // Lexicon-only stub: adjudication untouched (undefined slash methods are
     // never reached — these benches drive plain-draft flows only).
     ...(lex !== undefined
-      ? { slash: (() => ({ lexicon: () => lex })) as unknown as NonNullable<ShellDeps['slash']> }
+      ? {
+        slash: (() => ({
+          lexicon: { getSnapshot: () => lex, subscribe: () => () => {} },
+        })) as unknown as NonNullable<ShellDeps['slash']>,
+      }
       : {}),
   })
   if (over?.draft !== undefined && over.draft !== '') shell.setDraft(over.draft)