test: close coverage gaps from the recent rotation + Mercure work

Frontend (90.15→95.37 stmts / 91.83→97.01 lines):
  - useDeviceMercure: full composable test suite via a fake EventSource —
    open/merge/ignore-stale/parse-error/reconnect/dynamic-add/remove/
    no-op-when-unconfigured/cleanup-on-unmount.
  - HomeView: cover onTimePart's AM/PM and minute branches plus the
    nextPollExpectedAt-null fallback paths in the next-update preview.

Backend (no instrumentation before; pcov was already in the image,
just needed a <coverage> block in phpunit.dist.xml):
  - RotationService: one test per mode (NewestUpload, Random,
    LeastRecentlyShown), one for never-shown sorting first under LRS,
    and two for prioritizeNeverShown — narrows when never-shown exists,
    falls through to mode otherwise.
  - DeviceSerializer: contract test on the wire shape (REST + Mercure
    use the same serializer; silent rename here would break live updates
    instantly).
  - MercurePublisher: topic format + JSON encoding + the swallow-
    exceptions guarantee (a flaky hub must not break poll responses).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

phpunit.dist.xml

@@ -23,6 +23,13 @@
         </testsuite>
     </testsuites>
 
+    <!-- pcov ships with the php image; pass coverage flags on the CLI to enable. -->
+    <coverage pathCoverage="false">
+        <report>
+            <text outputFile="php://stdout" showOnlySummary="true"/>
+        </report>
+    </coverage>
+
     <source ignoreSuppressionOfDeprecations="true"
             ignoreIndirectDeprecations="true"
             restrictNotices="true"