feat(hud): render the Intel HUD from a store

IntelHUD no longer builds #intel-hud with innerHTML or writes its readouts
by id: it publishes its readouts, typing summary, REC blink, visibility,
layout variant and style colors through its publish option to the gev-hud
store, and the intel-hud application view renders them. The corners render
from mount so the panel layout controllers register them as obstacles.

The vessel layer presents the selected vessel's readout through
src/hudReadouts.js, which IntelHUD follows, instead of writing into the
HUD. The Cyber sonar state answers a HUD toggle from IntelHUD's visibility
rather than the class the view renders on its next update.

src/testSupport/hudReadouts.mjs

@@ -0,0 +1,35 @@
+// src/testSupport/hudReadouts.mjs — the Intel HUD's published readouts by the
+// element ids its view renders them in, for tests that drive IntelHUD alone.
+
+const FIELDS = Object.freeze({
+  'hud-alt': 'alt',
+  'hud-summary': 'summary',
+  'hud-mgrs': 'mgrs',
+  'hud-latlon': 'latlon',
+  'hud-bottom-line': 'bottom',
+  'hud-gsd': 'gsd',
+  'hud-coll': 'coll',
+  'hud-ona': 'ona',
+  'hud-mode': 'mode',
+  'hud-timestamp': 'timestamp',
+});
+
+/**
+ * A `publish` to hand IntelHUD and each readout's latest text.
+ * @returns {{publish: (view: object) => void, view: () => ?object,
+ *   elements: Map<string, {readonly textContent: string}>}}
+ */
+export function hudReadouts() {
+  let view = null;
+  const elements = new Map(
+    Object.entries(FIELDS).map(([id, field]) => [
+      id,
+      {
+        get textContent() {
+          return view?.[field] ?? '';
+        },
+      },
+    ]),
+  );
+  return { publish: (next) => (view = next), view: () => view, elements };
+}