fix(window-restore): re-read workspace/window state after the wofi pick

The picker blocks on user input for however long they take. In that
window niri can destroy a dynamic workspace (its last window closed)
and renumber every later one down, so the pre-pick current["idx"] can
point at the wrong workspace by the time move-window-to-workspace
runs — restoring to the wrong place. The stashed window itself can
also have closed in the meantime, in which case the move/focus calls
would silently no-op. main() now re-reads workspaces/windows after
pick() returns, recomputes the current workspace, and confirms the
chosen window still exists before acting; each bails with a notify
and the documented exit code otherwise. Adds window_exists() as a
pure, tested helper alongside the existing pure functions.

Also: clarify the focus=false comment in niri/config.kdl (it read as
if focus=false caused the problem it actually prevents), and note in
the design doc's Out of scope section that the stash's index-1
pinning is per-output, so a second monitor would need its own
analysis of the Mod+1..9 offset.

scripts/tests/window-restore.test.py

@@ -78,6 +78,11 @@ DUPES = [
 ]
 check("duplicate labels resolve to the first", wr.resolve(DUPES, "Alacritty — same")["id"], 20)
 
+# --- window_exists -----------------------------------------------------------
+check("window_exists is True for a present window", wr.window_exists(WINDOWS, 11), True)
+check("window_exists is False once the window has closed", wr.window_exists(WINDOWS, 999), False)
+check("window_exists is False for an empty list", wr.window_exists([], 11), False)
+
 # --- pick ------------------------------------------------------------------
 check("pick returns the chooser's selection",
       wr.pick(["a", "b"], lambda text: "b"), "b")