feat(vdisplay): Stage 5 layout foundation — arrangement engine + /display/layout + group placement

§6A layout, riding the Stages 1-3 registry with no protocol change:

- vdisplay/layout.rs: pure arrangement engine — auto-row (left-to-right in
  acquire order, top-aligned) + manual (per-identity-slot offsets, auto-row
  fallback for unpinned members). Unit-tested.
- Registry group model (Linux): group = backend (one desktop per compositor
  session). /display/state groups entries, orders by acquire (gen), and computes
  each member's position via the engine (pure `assemble_displays`, unit-tested).
  DisplayInfo carries group/display_index/position/identity_slot/topology. The
  backend reports its resolved slot via the new VirtualDisplay::last_identity_slot
  (KWin only), so the arrangement + state key on per-client identity.
- Registry-driven position apply: new VirtualDisplay::apply_position(x,y) (default
  no-op; KWin drives kscreen-doctor). Right after create the registry computes the
  new display's position over its whole group (pure `position_for_new`, unit-tested)
  and applies it — one seam for BOTH deterministic auto-row AND manual placement.
  Guarded: the origin (0,0) is skipped, so a single-display / first-of-group session
  (and every non-KWin backend) issues no positioning — the historical single-display
  path is unchanged. On-glass-validation-pending.
- PUT /api/v1/display/layout: persists the console's manual arrangement via the pure
  EffectivePolicy::with_manual_layout transform (locks current effective behavior
  into explicit Custom fields + sets a manual layout, so arranging is orthogonal to
  the other axes). OpenAPI regenerated.
- /display/settings `enforced` now lists all five axes (keep_alive, topology,
  mode_conflict [Stage 4], identity [Stage 3], layout [Stage 5]) — was stale at
  keep_alive+topology; the console reads it to know which controls are live.

Still Stage-5 TODO (design/display-management.md §11): Mutter/wlroots group-aware
analogues, per-group topology restore, the web arrangement table, gamescope decline.

cargo build/test/clippy/fmt green; OpenAPI in sync.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay/policy.rs

@@ -273,6 +273,29 @@ impl DisplayPolicy {
     }
 }
 
+impl EffectivePolicy {
+    /// Build a persistable `Custom` [`DisplayPolicy`] that keeps THIS effective behavior but replaces
+    /// the arrangement with a **manual** layout at `positions` — the `/display/layout` endpoint's
+    /// transform, factored out pure so arranging displays stays orthogonal to the other axes and is
+    /// unit-tested without touching the global store. (`Custom` so the explicit fields — incl. the new
+    /// layout — rule; a named preset would ignore them.)
+    pub fn with_manual_layout(&self, positions: BTreeMap<String, Position>) -> DisplayPolicy {
+        DisplayPolicy {
+            version: 1,
+            preset: Preset::Custom,
+            keep_alive: self.keep_alive,
+            topology: self.topology,
+            mode_conflict: self.mode_conflict,
+            identity: self.identity,
+            layout: Layout {
+                mode: LayoutMode::Manual,
+                positions,
+            },
+            max_displays: self.max_displays,
+        }
+    }
+}
+
 /// The field bundle a named preset expands to; `None` for [`Preset::Custom`]. The single expansion
 /// table — the docs' preset table mirrors this and the `presets_match_doc` test guards the shape.
 pub fn preset_fields(preset: Preset) -> Option<EffectivePolicy> {
@@ -526,6 +549,33 @@ mod tests {
         assert_eq!(p.max_displays, 16);
     }
 
+    #[test]
+    fn with_manual_layout_preserves_behavior_and_sets_positions() {
+        // Start from a preset's effective behavior (workstation: 5-min linger, exclusive, per-client).
+        let eff = DisplayPolicy {
+            preset: Preset::Workstation,
+            ..DisplayPolicy::default()
+        }
+        .effective();
+        let mut positions = BTreeMap::new();
+        positions.insert("1".to_string(), Position { x: 0, y: 0 });
+        positions.insert("7".to_string(), Position { x: 2560, y: 0 });
+        let p = eff.with_manual_layout(positions);
+        // Preset drops to Custom so the explicit fields (incl. the layout) rule…
+        assert_eq!(p.preset, Preset::Custom);
+        // …every other behavior axis is preserved verbatim…
+        assert_eq!(p.keep_alive, eff.keep_alive);
+        assert_eq!(p.topology, eff.topology);
+        assert_eq!(p.mode_conflict, eff.mode_conflict);
+        assert_eq!(p.identity, eff.identity);
+        assert_eq!(p.max_displays, eff.max_displays);
+        // …and the arrangement is the manual layout we asked for, surviving the effective round-trip.
+        let e2 = p.effective();
+        assert_eq!(e2.layout.mode, LayoutMode::Manual);
+        let want = Position { x: 2560, y: 0 };
+        assert_eq!(e2.layout.positions.get("7"), Some(&want));
+    }
+
     #[test]
     fn partial_json_fills_defaults() {
         // A hand-written file with only a couple of fields loads, the rest defaulting.