feat(site-builder): command registry, shortcut matcher, tree walkers and top-bar command menu

Library only; nothing is mounted. Adds minimal DeviceContext/NoticeContext
stubs (owned by W0-breakpoints / W0-registry-notices) that expose the
agreed interfaces.

Refs #39 #49 #34

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

craft/src/styles/editor.css

@@ -2054,3 +2054,47 @@ body {
   padding: 4px 12px;
 }
 .conflict-modal-actions button:disabled { opacity: 0.5; cursor: default; }
+
+/* W0-commands: topbar command menu */
+.topbar-command-menu { position: relative; display: inline-flex; }
+.topbar-command-btn {
+  background: transparent;
+  border: 1px solid transparent;
+  color: inherit;
+  border-radius: var(--radius-lg);
+  padding: 4px 10px;
+  cursor: pointer;
+}
+.topbar-command-btn:hover, .topbar-command-btn[aria-expanded="true"] { border-color: var(--color-border); }
+.topbar-command-dropdown {
+  position: absolute;
+  top: calc(100% + 6px);
+  right: 0;
+  z-index: var(--z-overflow, 1000);
+  min-width: 240px;
+  background: var(--color-bg-elevated);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-lg);
+  box-shadow: 0 8px 28px rgba(0, 0, 0, 0.5);
+  padding: 6px;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+.topbar-command-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 16px;
+  background: transparent;
+  border: 0;
+  color: inherit;
+  text-align: left;
+  padding: 8px 10px;
+  border-radius: var(--radius-md, 6px);
+  cursor: pointer;
+}
+.topbar-command-item:hover:not(:disabled) { background: var(--color-bg-hover); }
+.topbar-command-item:disabled { opacity: 0.45; cursor: default; }
+.topbar-command-shortcut { font-size: 12px; opacity: 0.6; }
+.topbar-command-empty { padding: 8px 10px; font-size: 12px; opacity: 0.6; }