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.rs

@@ -64,6 +64,23 @@ pub trait VirtualDisplay: Send {
     /// Default: no-op — only the Windows pf-vdisplay backend uses it (Linux compositors own their virtual
     /// output identity). `None` = anonymous/unpaired/GameStream → the backend's auto (slot-based) identity.
     fn set_client_identity(&mut self, _fingerprint: Option<[u8; 32]>) {}
+    /// The stable identity slot the backend resolved for the most recent [`create`](Self::create) —
+    /// the per-client id the identity policy assigned (`Some`), or `None` for shared/anonymous. The
+    /// registry reads it right after `create` to key the display's group **arrangement** (manual
+    /// per-slot positions) and to label the mgmt `/display/state` slot. Default `None`: a backend
+    /// with no per-client identity (Mutter/wlroots/gamescope) always auto-rows. Only KWin (per-slot
+    /// output naming) reports a real slot on Linux.
+    fn last_identity_slot(&self) -> Option<u32> {
+        None
+    }
+    /// Place the most-recently-[created](Self::create) output at `(x, y)` in the desktop coordinate
+    /// space (design `display-management.md` §6.2 — layout). The registry, which owns the display
+    /// **group**, computes the position from the whole group (auto-row or the console's manual
+    /// arrangement) and calls this right after `create`. Default no-op: only backends that can position
+    /// an output (KWin) implement it; the registry never calls it for the desktop origin `(0, 0)`, so a
+    /// single-display / first-of-group session issues no positioning at all. Best-effort — a failure
+    /// leaves the compositor's default placement.
+    fn apply_position(&mut self, _x: i32, _y: i32) {}
 }
 
 /// Compositors punktfunk knows how to drive (plan §6).
@@ -729,6 +746,11 @@ pub(crate) mod lifecycle;
 #[path = "vdisplay/registry.rs"]
 pub(crate) mod registry;
 
+// The pure display-arrangement engine (auto-row / manual → per-member positions), platform-neutral
+// and unit-tested; the registry (state readout) and the KWin position apply consume it.
+#[path = "vdisplay/layout.rs"]
+pub(crate) mod layout;
+
 /// Resolve a [`policy::Topology`] to a concrete value (never [`policy::Topology::Auto`]). `Auto`
 /// reproduces today's default: **extend** under an explicit `PUNKTFUNK_COMPOSITOR` pin (the CI/test
 /// posture, where the host isn't the sole desktop), else **exclusive** (Windows + the auto-detected