feat: v0.4.0 migrate to lib-core.panel persistent widgets

Migrate the top toolbar, right-side Layers panel, and bottom palette
strip to lib-core.panel v0.3.0 persistent widgets (W.2 of the
panel-window-manager plan):

  map-editor.toolbar -- top strip (full width, ~290 px tall)
  map-editor.layers  -- right side-panel (160 px wide)
  map-editor.palette -- bottom strip (full width, 64 px tall)

All three registered as persistent=true (open from module-init, not
dismissed by ESC), input_block='self' (hit-test claims clicks within
own bounds; misses fall through to canvas), and chromeless=true
(panel-lib skips its title-bar/border/X — widgets keep their own
visual style).

Existing draw_menu_bar / draw_side_panel / draw_palette and
handle_click logic is wrapped via thin widget render + handle_input
fns (M.draw_toolbar_widget / M.handle_toolbar_click etc.); no
behavioural change to the widget interiors.

Map-canvas remains module-rendered. Canvas drag-paint is now gated on
panel.point_in_any_panel(mx, my) so widget clicks no longer bleed
through into map paint.

Removed state.show_layers_panel / show_palette and their toggle
helpers (panel-lib manages widget visibility; Window menu items now
flip panel.is_open/toggle on the widget ids directly). Modals stay
module-rendered + module-routed (handled by handle_modal_click
before panel.update).

Manifest bumped 0.3.4 -> 0.4.0; lib-core.panel dep added at 0.3.0.

manifest.module

@@ -1,6 +1,6 @@
 {
     "id": "map-editor",
-    "version": "0.3.4",
+    "version": "0.4.0",
     "kind": "module",
     "api": "^0.1",
     "ci_frames": 30,
@@ -14,6 +14,7 @@
         {"id":"lib-core.composition","version":"0.3.0"},
         {"id":"lib-core.camera","version":"0.3.0"},
         {"id":"lib-core.input","version":"0.4.0"},
+        {"id":"lib-core.panel","version":"0.3.0"},
         {"id":"lib-asset.prototype-blob-geom","version":"0.1.0"}
     ]
 }