feat(vdisplay): KWin per-slot output naming for persistent scaling (Stage 3)

The KWin backend names its output Virtual-punktfunk-<id> from the client's
stable identity slot, so KWin persists per-output config (scale/mode) by name in
kwinoutputconfig.json and reapplies that client's scaling on reconnect — the KDE
scaling ask. Also fixes the latent clash where two concurrent sessions both used
Virtual-punktfunk (topology name-matching now uses the per-slot name).

- identity::global() + resolve_slot(fp, mode, default) — the shared persisted map
  (Windows manager dropped its own field; both use the global — never same-process).
  Default identity is per-platform: PerClient on Windows, Shared on Linux, so
  unconfigured hosts keep today's behavior (Linux = single 'punktfunk' name).
- KwinDisplay carries the client fp (set_client_identity), computes the per-slot
  name, threads it through the stream_virtual_output name + the topology helpers
  (set_custom_refresh / apply_virtual_primary[_only] / other_enabled_outputs).

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

crates/punktfunk-host/src/vdisplay/windows/manager.rs

@@ -169,10 +169,10 @@ pub(crate) struct VirtualDisplayManager {
     /// The current IDD-push session's stop flag; a new connection signals the prior one to release its
     /// monitor before the fresh one is created (was the `IDD_SESSION_STOP` global in `punktfunk1`).
     idd_session_stop: Mutex<Option<Arc<AtomicBool>>>,
-    /// Persistent per-client (cert-fingerprint) → stable monitor-id map. A monitor CREATE resolves the
-    /// connecting client's id here, so the client keeps the same EDID serial + IddCx ConnectorIndex across
-    /// reconnects and Windows reapplies its saved per-monitor config (DPI scaling). See [`super::identity`].
-    identity_map: Mutex<super::identity::DisplayIdentityMap>,
+    // The per-client stable monitor-id map is now the process-wide `super::identity::global()`
+    // (shared with the Linux KWin backend's per-slot naming — never same-process). A monitor CREATE
+    // resolves the client's id via `identity::resolve_slot`, so it keeps the same EDID serial + IddCx
+    // ConnectorIndex across reconnects and Windows reapplies its saved per-monitor DPI scaling.
 }
 
 static VDM: OnceLock<VirtualDisplayManager> = OnceLock::new();
@@ -188,7 +188,6 @@ pub(crate) fn init(driver: Box<dyn VdisplayDriver>) -> &'static VirtualDisplayMa
         state: Mutex::new(MgrState::Idle),
         setup_lock: Mutex::new(()),
         idd_session_stop: Mutex::new(None),
-        identity_map: Mutex::new(super::identity::DisplayIdentityMap::load()),
     })
 }
 
@@ -528,20 +527,13 @@ impl VirtualDisplayManager {
         // Resolve the connecting client's STABLE per-client monitor id (so Windows reapplies its saved
         // per-monitor config — DPI scaling — on reconnect); `None`/anonymous → 0 = the driver
         // auto-allocates the lowest-free id (the original slot-based behavior). The `identity` policy
-        // picks the key: per-client (fingerprint) or per-client-mode (fingerprint + resolution).
-        let per_client_mode = matches!(
-            crate::vdisplay::policy::prefs()
-                .configured_effective()
-                .map(|e| e.identity),
-            Some(crate::vdisplay::policy::Identity::PerClientMode)
-        );
-        let preferred_id = client_fp
-            .map(|fp| {
-                let key =
-                    super::identity::identity_key(fp, (mode.width, mode.height), per_client_mode);
-                self.identity_map.lock().unwrap().resolve(&key)
-            })
-            .unwrap_or(0);
+        // picks per-client vs per-client-mode; Windows defaults to PerClient (its historical behavior).
+        let preferred_id = super::identity::resolve_slot(
+            client_fp,
+            (mode.width, mode.height),
+            crate::vdisplay::policy::Identity::PerClient,
+        )
+        .unwrap_or(0);
         // SAFETY: `create_monitor`'s own `# Safety` contract guarantees `dev` is the live control
         // handle; we forward it unchanged to `add_monitor`, whose precondition is exactly that.
         // `resolve_render_pin()` returns an `Option<LUID>` by value (plain `Copy`), so no borrowed