1.4.3: database collector rewritten for Drupal 10/11 event API

The 1.3.0 collector implemented DatabaseInstrumentor as a subclass
of Drupal\Core\Database\Log and overrode log() — the Drupal 8/9
hook surface. Drupal 10 routed StatementWrapperIterator through
a modern event-based path (StatementExecutionEndEvent); the
legacy log() callback only fires when an external caller has
called Connection::setLogger(), and core's modern path no longer
does that. The collector compiled and wired cleanly but received
no queries in production.

1.4.1 fixed a real but unrelated priority-ordering bug that sat
under the API mismatch; the collector remained dead. Field-test
on 1.4.2 confirmed via direct introspection that the legacy
callback is never invoked under Drupal 11.3.8.

1.4.3 rewires to the modern API:

- DatabaseInstrumentor no longer extends Log. Implements
  EventSubscriberInterface and subscribes to
  Drupal\Core\Database\Event\StatementExecutionEndEvent.
  Counters use $event->getElapsedTime(). Listener filters by
  $event->key === 'default' so multi-DB sites don't see replica
  queries counted against per-request totals.

- DatabaseCollector::onRequest calls
  $connection->enableEvents([StatementExecutionEndEvent::class])
  at kernel.request when the config toggle is on. setLogger() is
  gone.

- Side benefit: the 1.3.0 trade-off "replaces other database
  loggers (devel, webprofiler) for the duration of the request"
  no longer applies. Events have multiple listeners; cwv now
  coexists with any other module tapping the same event. Form
  description and panel #empty text updated.

Adds DatabaseInstrumentorTest unit test (6 cases):
- testIsEventSubscriber — fails fast if anyone regresses the
  modern-API wiring. Catches the exact class of bug that
  shipped with 1.3.0.
- testListenerCountsQueriesWhenEnabled — counters increment on
  dispatched events, slow-query threshold honoured.
- testListenerSkipsWhenDisabled — zero counters when disabled.
- testListenerFiltersByConnectionKey — only 'default' counted.
- testResetZeroesCounters.
- testSlowQueryThresholdClampsToPositive.

cwv.database_instrumentor service tagged event_subscriber again
(1.4.1 removed it when diagnosing the priority bug; the modern
event listener needs it).

Library version bump 1.4.2 -> 1.4.3 (cache-busts the JS asset).
No schema change, no config-key change, no update hook. First
release where the database correlation panel actually populates
in production since the surface was introduced in 1.3.0.