feat(plan): device markers are landmarks in the Plan editor

The Plan editor hid every device marker (display: none), so walls,
openings and stairs were placed against devices from memory. It now
shows them exactly as the Background editor does (#362): the same
markers as View, at the Background editor's effective opacity, fully
pointer-inert. The fade is filter: opacity(0.35) on each marker, not
on .devlayer, because room labels and room settings buttons share that
layer and stay opaque and interactive; filter multiplies the marker's
own opacity (.unavail -> 0.35 x 0.35), as the Background layer does.
Marker handlers already fail closed outside View/Devices.

demo/smoke_plan_device_landmarks.mjs proves AC1/AC2: same markers as
View, per-marker alpha equal to Background, core and capsule fall
through, a real click on a marker reaches the Walls tool and a room
settings button below, cursor and target unchanged, handlers inert, no
tabindex. Three mutants guard hiding, opacity override and the pointer
boundary. UX-MODES, DECOR-EDITOR, USER-GUIDE.ru mode table, changelogs.

Issue: #687
User-Visible: yes
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

scripts/mutation-registry.mjs

@@ -3955,6 +3955,51 @@ const MUTANT_DEFINITIONS = [
       replace: '          && !inEditorSecondary) {',
     }],
   },
+  {
+    id: 'plan-device-landmarks-hidden',
+    guard: 'node demo/smoke_plan_device_landmarks.mjs',
+    because: '#687 AC1: the Plan editor shows the same markers as View as landmarks; hiding '
+      + 'them again removes the reference the owner asked for',
+    patches: [{
+      file: 'src/styles/plan.styles.ts',
+      find: '    .stage.markup .devlayer .dev {\n'
+        + '      filter: opacity(0.35);\n'
+        + '    }\n',
+      replace: '    .stage.markup .devlayer .dev {\n'
+        + '      display: none;\n'
+        + '    }\n',
+    }],
+  },
+  {
+    id: 'plan-device-landmarks-override-own-opacity',
+    guard: 'node demo/smoke_plan_device_landmarks.mjs',
+    because: '#687 contract item 2: the fade multiplies the marker\'s own opacity as the '
+      + 'Background layer does; an opacity override shows an unavailable marker at 35% '
+      + 'instead of 35% x 35%',
+    patches: [{
+      file: 'src/styles/plan.styles.ts',
+      find: '    .stage.markup .devlayer .dev {\n'
+        + '      filter: opacity(0.35);\n',
+      replace: '    .stage.markup .devlayer .dev {\n'
+        + '      opacity: 0.35;\n',
+    }],
+  },
+  {
+    id: 'plan-device-landmarks-hit-target',
+    guard: 'node demo/smoke_plan_device_landmarks.mjs',
+    because: '#687 AC2: a visible marker must never own a Plan-editor point; without the '
+      + 'pointer boundary its 44 px hit area and capsule swallow the Walls tool click and the '
+      + 'room settings button below',
+    patches: [{
+      file: 'src/styles/plan.styles.ts',
+      find: '    .stage.markup .devlayer .dev,\n'
+        + '    .stage.markup .devlayer .dev *,\n'
+        + '    .stage.markup .devlayer .dev::before {\n'
+        + '      pointer-events: none;\n'
+        + '    }\n',
+      replace: '',
+    }],
+  },
   {
     id: 'opening-light-quantum-identity',
     guard: 'node --test --test-name-pattern="#366" test/logic.test.mjs',