move pan input into an InputMapper

First slice of pulling key handling out of GameWorldView. The widget no longer
holds A/D key state; the mapper owns it and publishes the resulting direction as
PanDirectionChangedEvent, which the view consumes like any other event.

The event is level-triggered on purpose — the payload is the complete current
direction, PanDirection::None included — so a receiver never reconstructs state
from edges and cannot be left panning by a missing key-up. It is also the one
place in the UI where caching an event payload is right rather than wrong: input
has no other authority to re-read from, so the mapper is the source of truth.
Both points are written down on the event, since they look like violations of
the surrounding conventions otherwise.

Holding the state in one object is what makes releaseAll() possible; restart
uses it, and it is what a focusOutEvent will call to fix the stuck-pan bug in a
follow-up.

Bindings stay hard-coded. Only the ownership moved, so behaviour is unchanged,
including both keys held cancelling out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JcReq7hVk4KUPhTDKWAG7K

src/ui/InputMapper.cpp

@@ -0,0 +1,70 @@
+#include "InputMapper.h"
+
+#include <memory>
+
+#include <QKeyEvent>
+
+#include "EventManager.h"
+#include "PanDirectionChangedEvent.h"
+
+bool InputMapper::handleKeyPress(QKeyEvent* event)
+{
+    // Auto-repeat says nothing new about which keys are down, and a held action is
+    // already held.
+    if (event->isAutoRepeat()) { return false; }
+
+    switch (event->key())
+    {
+    case Qt::Key_A:
+        m_panLeftHeld = true;
+        updatePanDirection();
+        return true;
+    case Qt::Key_D:
+        m_panRightHeld = true;
+        updatePanDirection();
+        return true;
+    default:
+        return false;
+    }
+}
+
+bool InputMapper::handleKeyRelease(QKeyEvent* event)
+{
+    if (event->isAutoRepeat()) { return false; }
+
+    switch (event->key())
+    {
+    case Qt::Key_A:
+        m_panLeftHeld = false;
+        updatePanDirection();
+        return true;
+    case Qt::Key_D:
+        m_panRightHeld = false;
+        updatePanDirection();
+        return true;
+    default:
+        return false;
+    }
+}
+
+void InputMapper::releaseAll()
+{
+    m_panLeftHeld  = false;
+    m_panRightHeld = false;
+    updatePanDirection();
+}
+
+void InputMapper::updatePanDirection()
+{
+    // Holding both keys cancels out rather than favouring one.
+    PanDirection direction = PanDirection::None;
+    if (m_panLeftHeld != m_panRightHeld)
+    {
+        direction = m_panLeftHeld ? PanDirection::Left : PanDirection::Right;
+    }
+
+    if (direction == m_panDirection) { return; }
+    m_panDirection = direction;
+    EventManager::getInstance()->sendEventImmediately(
+        std::make_shared<PanDirectionChangedEvent>(direction));
+}