feat(vdisplay): Linux keep-alive pool — registry-owned display lifecycle (Stage 1b)

The ownership split (design/display-management.md §3): the registry owns the
per-session virtual-display lifecycle on Linux, so a display can outlive its
session (keep-alive) and be reused on reconnect.

- registry.rs: a Linux pool driven by the pure lifecycle machine. acquire()
  reuses a kept (lingering/pinned) display of the same backend+mode, else
  creates one and keeps the backend's keepalive so the compositor output (and
  its PipeWire node_id) survives the session. The session's capturer holds a
  gen-stamped DisplayLease instead of the real keepalive; its drop drives
  linger/teardown. Enabling fact: KWin/Mutter/gamescope put their node on the
  DEFAULT PipeWire daemon (remote_fd=None) — reconnect re-attaches by node_id,
  no fd re-open. wlroots (remote_fd=Some, xdpw portal) passes through unchanged
  (teardown-on-drop) pending the fresh-portal-capture re-attach.
- Default (unconfigured) linger = Immediate → today's teardown-on-disconnect,
  so no behavior change without a keep-alive policy; concurrent sessions still
  each create their own output (reuse only matches LINGERING entries).
- Wired build_pipeline (punktfunk1) + gamestream through registry::acquire;
  capture_virtual_output signature unchanged. Windows delegates to vd.create
  (the manager already leases) — unchanged.

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

crates/punktfunk-host/src/punktfunk1.rs

@@ -3508,7 +3508,11 @@ fn build_pipeline(
     bit_depth: u8,
     plan: crate::session_plan::SessionPlan,
 ) -> Result<Pipeline> {
-    let vout = vd.create(mode).context("create virtual output")?;
+    // Acquire through the registry (design/display-management.md): on Linux this pools the display
+    // for keep-alive (reuse a kept one, or create + keep the backend's keepalive so it outlives the
+    // session per policy); on Windows it delegates to `vd.create` (the manager already leases). The
+    // returned `VirtualOutput`'s keepalive is a registry lease — the capturer holds it as before.
+    let vout = crate::vdisplay::registry::acquire(vd, mode).context("create virtual output")?;
     // The backend reports the refresh it actually achieved in `preferred_mode.2` (KWin may cap a
     // virtual output at 60 Hz if the custom-mode install was rejected). Pace the encoder + frame
     // clock to that, not the requested rate, so we don't emit phantom duplicate frames over a