Add global keyboard EventRouter and unify publish (M6, §01D)

One module owns the document keydown listener and dispatches the shortcut map
(Cmd/Ctrl+Shift+N / +K / +S / +, / Escape), platform-aware, via the single-source
focus-utils interactive-context gate. Escape and Cmd/Ctrl+S run before the gate so
save works while editing; the rest are suppressed mid-typing. Publish + its toast
move into services/snippet-actions so the button and Cmd/Ctrl+S behave identically.
Removes the ad-hoc keydown handler from App and Monaco's Cmd+S command.

docs/architecture/10-interaction-and-feedback.md

@@ -65,6 +65,13 @@ nature of the message, not by convenience.
   _inline on the control_ ("Copied") with a polite `aria-live` announcement for assistive
   tech, never a toast-per-copy. This refines the spec's earlier blanket "every action
   toasts" (spec §01F/§02/§05, reconciled).
+- **An action's confirmation belongs with the action, not its call site.** When an action is
+  reachable from more than one trigger (e.g. publish is both a toolbar button _and_
+  Cmd/Ctrl+S), pair the store mutation and its toast in **one `services/` helper**
+  (`publishActiveSnippet`) that every trigger calls — otherwise the toast rides one path and
+  the other publishes silently (the exact inconsistency this rule prevents). The shortcut is
+  owned globally by the EventRouter (arch 04), so the helper is the only place the outcome is
+  confirmed.
 - **The same failure can light up two channels.** An unrenderable spec shows the _same_
   message inline in both the editor (§03E) and the preview (§04) — one producer
   (`PreviewStore`), two subscribers. That's intentional, not duplication.