Surface VRAM shortfall to the user and let them unload models

When the geometry in view needs more GPU memory than the cache can
hold, the viewer keeps the largest on-screen chunks resident and streams
the rest as the camera moves. That is the right degradation, but it was
invisible: nothing told the user the scene did not fit, and the only
lever was removing or hiding models, neither of which is "keep it in the
federation but stop spending GPU memory on it".

Viewer core:
- ModelGpuData::unloaded, with drawable() = !hidden && !unloaded now the
  test every cull / draw / pick / streaming pass uses. unloadModel evicts
  every chunk and releases the model's own buffers; loadModel recreates
  them from the CPU mirrors (no disk read) and lets chunks stream back.
  Recompose keeps the CPU instances current while a model is unloaded so
  a reload sees up-to-date transforms. The MeshGpu/InstanceGpu record
  builders are factored out so load and reload share them.
- FrameStats reports the camera's working set: chunks wanted, how many
  of those are not resident, and their bytes.
- modelVramBytes / isModelUnloaded accessors, forwarded by ViewportWindow.

BonsaiViewer:
- Models tree gains a memory column (name | MB | eye) refreshed once a
  second and on load-state changes; unloaded models read "unloaded" in
  italics. The viewport stays the single authority for the state;
  SessionState only carries the modelLoadStateChanged notification.
- Context menu: "Unload Model" / "Load Model", distinct from hide and
  remove, reporting the MB freed in the status bar.
- Status bar notice, independent of the perf-stats toggle, once the
  shortfall has persisted for 3 s (a moment of missing chunks after any
  camera move is normal): "GPU memory full: N of M visible chunks (X MB)
  not loaded", with a tooltip pointing at Unload. The perf label also
  shows "N/M chunks waiting".

Verified on the GPU: unloading a 497 MB model frees it immediately with
the others still rendering; reloading streams all 180 chunks back.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/bonsaiviewer/modules/models/Panel.cpp

@@ -28,6 +28,7 @@
 #include "../../components/Section.h"
 #include "../../components/SvgIcon.h"
 #include "../../../ifcviewer/Federation.h"
+#include "../../../ifcviewer/ViewportWindow.h"
 
 #include <QDataStream>
 #include <QDrag>
@@ -79,6 +80,7 @@ QStringList selectedModelIdsAt(QTreeView* tree, const QModelIndex& clicked_index
 }
 
 constexpr int kVisibilityColumnWidth = 28;
+constexpr int kMemoryColumnWidth     = 72;  // "1234 MB" / "unloaded"
 
 // QTreeView subclass that handles drag-and-drop. Drop logic dispatches
 // through commands (not directly into the model) so notifications + status
@@ -250,7 +252,7 @@ ModelsPanel::ModelsPanel(bonsaiviewer::SessionState* session_state,
 
     connect(tree_, &QTreeView::clicked, this, [this](const QModelIndex& index) {
         if (!index.isValid()) return;
-        if (index.column() == 1) {
+        if (index.column() == VisibilityColumn) {
             commands::toggleVisibility(*session_state_, kindOf(index), idOf(index));
             return;
         }
@@ -380,6 +382,24 @@ ModelsPanel::ModelsPanel(bonsaiviewer::SessionState* session_state,
                 commands::saveModelAsToCloud(*session_state_, *this, id);
             });
 
+            // GPU residency. Unload keeps the model in the federation (and
+            // its visibility) but frees everything it holds on the GPU — the
+            // lever when the scene does not fit in VRAM. Load brings it back.
+            menu.addSeparator();
+            const uint32_t session_model_id = session_state_->sessionModelIdForModelId(id);
+            const bool unloaded = session_model_id != 0 && viewport_->isModelUnloaded(session_model_id);
+            QAction* residency = menu.addAction(
+                components::icons::makeSvgIcon(":/icons/cube.svg"),
+                unloaded ? "Load Model" : "Unload Model");
+            residency->setEnabled(session_model_id != 0);
+            residency->setToolTip(unloaded
+                ? "Allocate GPU memory for this model again and stream its geometry back in."
+                : "Free this model's GPU memory while keeping it in the federation.");
+            connect(residency, &QAction::triggered, this, [this, id, unloaded]() {
+                if (unloaded) commands::loadModel(*session_state_, *viewport_, id);
+                else          commands::unloadModel(*session_state_, *viewport_, id);
+            });
+
             menu.addSeparator();
             QAction* remove = menu.addAction(
                 components::icons::makeSvgIcon(":/icons/minus-square.svg"), "Remove Model");
@@ -409,14 +429,16 @@ void ModelsPanel::setModel(FederationItemModel* model) {
 }
 
 void ModelsPanel::applyColumnLayout() {
-    // Column 0 (name) stretches to fill; column 1 (visibility icon) is fixed.
+    // The name stretches to fill; memory and visibility are fixed.
     QHeaderView* header = tree_->header();
-    if (header->count() < 2) return;
+    if (header->count() < ColumnCount) return;
     header->setStretchLastSection(false);
     header->setMinimumSectionSize(kVisibilityColumnWidth);
-    header->setSectionResizeMode(0, QHeaderView::Stretch);
-    header->setSectionResizeMode(1, QHeaderView::Fixed);
-    header->resizeSection(1, kVisibilityColumnWidth);
+    header->setSectionResizeMode(NameColumn, QHeaderView::Stretch);
+    header->setSectionResizeMode(MemoryColumn, QHeaderView::Fixed);
+    header->resizeSection(MemoryColumn, kMemoryColumnWidth);
+    header->setSectionResizeMode(VisibilityColumn, QHeaderView::Fixed);
+    header->resizeSection(VisibilityColumn, kVisibilityColumnWidth);
 }
 
 } // namespace bonsaiviewer::modules::models