feat(vdisplay): platform-neutral identity map + per-client-mode (Stage 3)

Generalize the Windows-only per-client stable-id map into vdisplay/identity.rs:
- DisplayIdentityMap keyed on a composable string (identity_key: fingerprint,
  or fingerprint+resolution under per-client-mode); LRU at 15, persisted to
  display-identity.json (migrated from the legacy pf-vdisplay-identity.json).
- Windows manager wired to it, picking the key from the identity policy.
- Foundation for KWin per-slot output naming (persistent KDE scaling) — the
  KWin wiring is the next Stage-3 step (needs a KWin box).
- Unit-tested (stable, per-client-mode split, LRU, key composition).

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

crates/punktfunk-host/src/vdisplay.rs

@@ -779,8 +779,11 @@ pub fn effective_topology() -> policy::Topology {
 #[cfg(target_os = "linux")]
 #[path = "vdisplay/linux/gamescope.rs"]
 mod gamescope;
-#[cfg(target_os = "windows")]
-#[path = "vdisplay/windows/identity.rs"]
+// Platform-neutral per-client stable display-id map (Stage 3): Windows seeds the monitor EDID +
+// ConnectorIndex from the id; KWin names its output from it. `allow(dead_code)` because only Windows
+// consumes it in non-test code today — the KWin wiring is the next Stage-3 step.
+#[allow(dead_code)]
+#[path = "vdisplay/identity.rs"]
 pub(crate) mod identity;
 #[cfg(target_os = "linux")]
 #[path = "vdisplay/linux/kwin.rs"]