fix(display): define the display-server messages once, in src/shared

The display controller and the viewer routes each described the
heartbeat their own way and disagreed about one thing: the display sent
null for "nothing to report", and the route accepted only a missing
field. Every heartbeat from a screen showing a native image was refused
with a 400. The roster therefore marked working screens Offline 90 s
after they opened, and never recorded offlineReady or cachedAssetCount.

src/shared/display-protocol.ts now defines every message between a
display and the server:
- viewer session request and response;
- heartbeat request and response;
- channel state;
- the realtime display payload.

Both halves import it. Its rules: a nullable field accepts null
explicitly, an absent field means "not reported", and an unknown field
is ignored, so a newer display is never refused by an older server.

- The routes parse with the shared parsers, and a refusal names the
  field with Signage's sanitized code.
- The server's channel state extends the wire type, so a field the
  display relies on cannot drift without a compile error.
- The display builds its heartbeat as the shared type, and reads its
  registration with the shared parser instead of casting it.
- Test stubs that described server responses more loosely than the
  server sends them (no viewerName, no stale, partial media) now match
  the real responses and are typed as the protocol.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/server/display/state.ts

@@ -1,4 +1,9 @@
 import type { ModuleStorage } from "@ekklesia/module-sdk";
+import type {
+  DisplayChannelState as DisplayChannelStateWire,
+  DisplaySelection,
+  DisplayTimeline,
+} from "../../shared/display-protocol.js";
 import {
   resolveNativeScene,
   SIGNAGE_NATIVE_PROVIDER_ID,
@@ -35,78 +40,21 @@ import type {
  * and a display refreshes state long before one lapses.
  */
 
-export interface DisplayMedia {
-  readonly url: string;
-  readonly expiresAtMs: number;
-  readonly width: number;
-  readonly height: number;
-}
+export type {
+  DisplayMedia,
+  DisplaySelection,
+  DisplayTimeline,
+  DisplayTimelineSlot,
+} from "../../shared/display-protocol.js";
 
-export interface DisplaySelection {
-  readonly id: string;
-  readonly revision: number;
-  readonly slotId: string | null;
-  readonly providerId: string;
-  readonly itemId: string;
-  readonly itemRevision: string;
-  readonly startedAtMs: number | null;
-  readonly deadlineAtMs: number | null;
-  readonly durationMs: number;
-  readonly availableUntilMs: number | null;
-  readonly withdrawn: boolean;
-  readonly stale: boolean;
-  /** `native-image` is drawn by Signage's own display code; `contribution` is mounted through the
-   * platform's contribution ABI by id. A display never receives a URL for either. */
-  readonly renderer:
-    | { readonly kind: "native-image" }
-    | {
-        readonly kind: "contribution";
-        readonly contributionId: string;
-        readonly contributionVersion: string;
-      }
-    | { readonly kind: "unavailable" };
-  /** The provider's own payload for this channel's audience, or its default. */
-  readonly payload: unknown;
-  readonly presentation: unknown;
-  /** Native images only: fresh delivery URLs, and the already-resolved scene. */
-  readonly media?: { readonly display: DisplayMedia; readonly thumbnail: DisplayMedia };
-  readonly scene?: ReturnType<typeof resolveNativeScene>;
-}
-
-export interface DisplayTimelineSlot {
-  readonly id: string;
-  readonly position: number;
-  readonly enabled: boolean;
-  readonly eligibleFromMs: number | null;
-  readonly eligibleUntilMs: number | null;
-  readonly estimatedDurationMs: number;
-}
-
-export interface DisplayTimeline {
-  readonly id: string;
-  readonly revision: number;
-  readonly eligibleFromMs: number | null;
-  readonly eligibleUntilMs: number | null;
-  readonly slots: readonly DisplayTimelineSlot[];
-}
-
-export interface DisplayChannelState {
-  readonly serverNowMs: number;
-  readonly channel: {
-    readonly id: string;
-    readonly key: string;
-    readonly name: string;
-    readonly groupId: string;
-  };
-  readonly crossfadeMs: number;
+/**
+ * The channel state the server builds: the wire shape a display reads (`src/shared`), plus the
+ * channel's name and the group's whole playback snapshot. Declared as an extension so that a field
+ * the display relies on cannot be renamed or retyped here without failing to compile.
+ */
+export interface DisplayChannelState extends DisplayChannelStateWire {
+  readonly channel: DisplayChannelStateWire["channel"] & { readonly name: string };
   readonly playback: PlaybackSnapshot;
-  readonly timeline: DisplayTimeline | null;
-  readonly current: DisplaySelection | null;
-  /** The held image, already resolved as a scene. Separate from `current` because a static hold sits
-   * *over* a preserved cursor rather than replacing it. */
-  readonly staticHold: DisplaySelection | null;
-  /** Bounded, newest first. Material for offline continuation, never a substitute for authority. */
-  readonly selections: readonly DisplaySelection[];
 }
 
 /** How many resolved selections travel to a display. Enough to cover a pattern and then some, small