feat: P3 — WebKit container process pinned to dashboard

Dashward now actually renders content on the dashboard workspace. The
container is a separate gjs subprocess (so a WebKit crash can't take
down gnome-shell), and the extension pins its window to the dashboard
slot via the WindowGuard whitelist.

container/src/main.ts: tiny GJS bootstrap that creates a borderless
fullscreen Gtk.Window with a WebKit2 WebView and loads dashboard.html
from a runtime directory passed in via argv. `GLib.set_prgname` happens
before any GTK init so Wayland's xdg-shell app_id matches
`top.hangmanlab.dashward.container` -- that's the wm_class fingerprint
the extension matches against.

extension/src/container-supervisor.ts: spawn the container via
Gio.Subprocess; pump its stdout/stderr into journal under `[container
stdout|stderr]` tags so we can diagnose WebKit crashes without
attaching; watch display::window-created for the app_id match; on
arrival, whitelist with WindowGuard before moving to dashboard (so the
move's window-added doesn't bounce); make_fullscreen; clear the cached
ref on the window's `unmanaged` signal. Dispose SIGTERMs the
subprocess. P3 explicitly skips auto-restart / exponential backoff and
DBus signaling -- those land in P4.

container/runtime/runtime.ts + styles.css: a "Dashward" placeholder
card on the 12-column grid so the dashboard workspace is visually
distinct from a regular workspace; widget mounting / edit mode is P5+.

Verified on the ubuntu2504-test VM: extension enables cleanly, dashboard
shows the placeholder, switching to/from dashboard works, ding's window
is still ignored. MESA/EGL stderr lines are VM-only software-rendering
fallback noise (no virgl).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/runtime/runtime.ts

@@ -1,6 +1,7 @@
-// Stub: page-side runtime (design §11).
-// Loads layout.json, instantiates widgets in iframes on the 12-column grid,
-// handles edit mode, persists changes via the shell bridge.
+// Page-side runtime (design §11). For P3 the grid is just decoration —
+// it renders a "no widgets yet" placeholder so the dashboard workspace
+// has visible content. Real widget loading / edit mode / layout
+// persistence come in P5+.
 
 declare global {
   interface Window {
@@ -10,6 +11,18 @@ declare global {
   }
 }
 
-console.info('[dashward-runtime] P0 stub loaded');
+const grid = document.getElementById('grid');
+if (grid) {
+  const placeholder = document.createElement('section');
+  placeholder.className = 'placeholder';
+  placeholder.innerHTML = `
+    <h1>Dashward</h1>
+    <p class="hint">P3 — empty dashboard.</p>
+    <p class="meta">No widgets installed yet. Widget SDK + edit mode arrive in P5.</p>
+  `;
+  grid.appendChild(placeholder);
+}
+
+console.info('[dashward-runtime] P3 placeholder mounted');
 
 export {};