refactor: single-source display lifecycle + scenario test no React

Move activeDisplay lifecycle to shared (one path, not duplicated in App):
- activateDisplay({campaignId, encounterId})
- clearDisplay()
- toggleHidePlayerHp(current)
All 6 App sites wired (start/end/2×delete-encounter/delete-campaign/hp-toggle).
Existing {patch,log} return shape preserved. No interface changes.

Combat.scenario.test.js rewritten to call shared directly, no React:
- Same actions as old UI version (roster chars, monsters, addAll, hp-toggle×2,
  start, 100 rounds of damage/heal/conditions/toggle/edit/deathsave/pause/
  resume/add/remove, end). Same funcs, same order, same data.
- Models two firestore docs (encounter + activeDisplay) via shared patches.
- Run time: 72s -> 4ms.

Both suites green: shared 91/91, App 77/77.

shared/turn.js

@@ -552,6 +552,27 @@ function endEncounter(encounter) {
   };
 }
 
+// ----------------------------------------------------------------------------
+// Display lifecycle (activeDisplay/status doc). Pure patches, same shape as
+// App's inline storage.updateDoc(getPath.activeDisplay(), ...). Single source.
+// Callers persist patch to the activeDisplay/status doc.
+// ----------------------------------------------------------------------------
+
+// ACTIVATE_DISPLAY — start combat: point player display at this encounter.
+function activateDisplay({ campaignId, encounterId }) {
+  return { patch: { activeCampaignId: campaignId, activeEncounterId: encounterId } };
+}
+
+// CLEAR_DISPLAY — end combat: player display shows nothing.
+function clearDisplay() {
+  return { patch: { activeCampaignId: null, activeEncounterId: null } };
+}
+
+// TOGGLE_HIDE_PLAYER_HP — flip player-HP visibility on display.
+function toggleHidePlayerHp(currentValue) {
+  return { patch: { hidePlayerHp: !currentValue } };
+}
+
 module.exports = {
   DEFAULT_MAX_HP,
   DEFAULT_INIT_MOD,
@@ -579,4 +600,7 @@ module.exports = {
   toggleCondition,
   reorderParticipants,
   endEncounter,
+  activateDisplay,
+  clearDisplay,
+  toggleHidePlayerHp,
 };