feat(vdisplay): lifecycle state machine + display state/release API (Stage 1)

Stage 1 of design/display-management.md — the lifecycle core + the display
management surface:

- vdisplay/lifecycle.rs: pure per-slot state machine (Idle/Active{refs}/
  Lingering{until}/Pinned) with acquire/release/expiry/force-release
  transitions. No I/O, no OS types — the platform-neutral distillation of the
  Windows manager's model. Unit + a 200k-iteration seeded property walk
  (no leaks / double-frees / refcount underflow across arbitrary interleavings).
- vdisplay/registry.rs: neutral snapshot/release facade over the per-OS
  lifecycle owners. Windows reads/controls the VirtualDisplayManager; Linux
  keep-alive (a per-session pool) lands in a following increment (needs GPU-box
  validation).
- windows/manager.rs: additive snapshot() + force_release() (no behavior change
  to the on-glass-validated path).
- mgmt: GET /api/v1/display/state (live/kept displays) + POST /api/v1/display/release
  (tear down lingering/pinned now; refuses active). OpenAPI regenerated.
- web console: Virtual displays card gains a live-display list (polled) with
  per-row + release-all buttons and a linger countdown.

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

web/messages/de.json

@@ -68,6 +68,15 @@
 	"display_save": "Speichern",
 	"display_effective": "Aktiv",
 	"display_pending_note": "Konfliktbehandlung, Identität und Layout werden gespeichert, aber noch nicht angewendet — sie folgen in späteren Versionen.",
+	"display_live": "Aktive Displays",
+	"display_none_live": "Derzeit keine virtuellen Displays.",
+	"display_state_active": "Aktiv",
+	"display_state_lingering": "Wird gehalten",
+	"display_state_pinned": "Angeheftet",
+	"display_release_btn": "Freigeben",
+	"display_release_all": "Alle gehaltenen freigeben",
+	"display_expires_in": "Abbau in {sec}s",
+	"display_sessions": "{count} streamend",
 	"clients_title": "Gekoppelte Geräte",
 	"clients_empty": "Noch keine gekoppelten Geräte.",
 	"clients_name": "Name",

web/messages/en.json

@@ -68,6 +68,15 @@
 	"display_save": "Save",
 	"display_effective": "In effect",
 	"display_pending_note": "Conflict handling, identity, and layout are stored but not enforced yet — they arrive in later releases.",
+	"display_live": "Live displays",
+	"display_none_live": "No virtual displays right now.",
+	"display_state_active": "Active",
+	"display_state_lingering": "Lingering",
+	"display_state_pinned": "Pinned",
+	"display_release_btn": "Release",
+	"display_release_all": "Release all kept",
+	"display_expires_in": "tears down in {sec}s",
+	"display_sessions": "{count} streaming",
 	"clients_title": "Paired clients",
 	"clients_empty": "No paired clients yet.",
 	"clients_name": "Name",

web/src/sections/Host/DisplayCard.tsx

@@ -2,10 +2,14 @@ import { useQueryClient } from "@tanstack/react-query";
 import { Button } from "@unom/ui/button";
 import { type FC, useEffect, useState } from "react";
 import {
+	getGetDisplayStateQueryKey,
 	getGetDisplaySettingsQueryKey,
 	useGetDisplaySettings,
+	useGetDisplayState,
+	useReleaseDisplay,
 	useSetDisplaySettings,
 } from "@/api/gen/display/display";
+import type { ApiDisplayInfo } from "@/api/gen/model";
 import { ApiError } from "@/api/fetcher";
 import type {
 	DisplayPolicy,
@@ -70,11 +74,100 @@ export const DisplaySection: FC = () => {
 						/>
 					)}
 				</QueryState>
+				<LiveDisplays />
 			</CardContent>
 		</Card>
 	);
 };
 
+/**
+ * The host's live/kept virtual displays, polled from `/display/state`, each with a Release button
+ * for lingering/pinned ones (active displays can't be released — that's session control).
+ */
+const LiveDisplays: FC = () => {
+	const qc = useQueryClient();
+	const state = useGetDisplayState({ query: { refetchInterval: 2_000 } });
+	const release = useReleaseDisplay();
+	const displays = state.data?.displays ?? [];
+	const kept = displays.filter((d) => d.state !== "active");
+
+	const doRelease = (slot?: number) =>
+		release.mutate(
+			{ data: { slot: slot ?? null } },
+			{ onSuccess: () => qc.invalidateQueries({ queryKey: getGetDisplayStateQueryKey() }) },
+		);
+
+	return (
+		<div className="space-y-2 border-t pt-4">
+			<div className="flex items-center justify-between gap-4">
+				<h4 className="text-sm font-medium">{m.display_live()}</h4>
+				{kept.length > 0 && (
+					<Button
+						size="sm"
+						variant="outline"
+						disabled={release.isPending}
+						onClick={() => doRelease()}
+					>
+						{m.display_release_all()}
+					</Button>
+				)}
+			</div>
+			{displays.length === 0 ? (
+				<p className="text-sm text-muted-foreground">{m.display_none_live()}</p>
+			) : (
+				<ul className="divide-y rounded-md border">
+					{displays.map((d) => (
+						<DisplayRow
+							key={d.slot}
+							d={d}
+							busy={release.isPending}
+							onRelease={() => doRelease(d.slot)}
+						/>
+					))}
+				</ul>
+			)}
+		</div>
+	);
+};
+
+const DisplayRow: FC<{ d: ApiDisplayInfo; busy: boolean; onRelease: () => void }> = ({
+	d,
+	busy,
+	onRelease,
+}) => {
+	const active = d.state === "active";
+	const stateLabel =
+		d.state === "active"
+			? m.display_state_active()
+			: d.state === "pinned"
+				? m.display_state_pinned()
+				: m.display_state_lingering();
+	return (
+		<li className="flex items-center justify-between gap-4 px-4 py-3">
+			<div className="min-w-0">
+				<div className="flex flex-wrap items-center gap-2">
+					<span className="font-medium">{d.mode}</span>
+					<Badge variant={active ? "success" : "secondary"}>{stateLabel}</Badge>
+					{active && d.sessions > 0 && (
+						<Badge variant="outline">{m.display_sessions({ count: d.sessions })}</Badge>
+					)}
+				</div>
+				<code className="text-xs text-muted-foreground">
+					{d.backend}
+					{d.expires_in_ms != null
+						? ` · ${m.display_expires_in({ sec: Math.ceil(d.expires_in_ms / 1000) })}`
+						: ""}
+				</code>
+			</div>
+			{!active && (
+				<Button size="sm" variant="outline" disabled={busy} onClick={onRelease}>
+					{m.display_release_btn()}
+				</Button>
+			)}
+		</li>
+	);
+};
+
 /** The server's `{ error }` message from a thrown `ApiError` (its `.data` body), for inline display. */
 const apiErrorMessage = (err: unknown): string | undefined => {
 	if (err instanceof ApiError) {