ifcviewer: per-element visibility (H / Shift+H / Alt+H)

Adds VisibilityState, a CPU-only sibling to SelectionState.  It owns
the canonical hidden-id set plus a flat per-object_id byte vector that
the cull's hot path queries inline (bounds check + byte load + compare
per surviving instance).  Hidden elements never reach the visible[]
SSBO so they don't draw or pick — matching Blender/CAD convention.

ViewportWindow registers every streamed and sidecar-cached object_id
with the new state, resets it on clearScene, and connects the changed
signal to invalidate cached cull state.  Three convenience verbs:
hideSelectedElements (union into hidden), isolateSelectedElements
(replace hidden with live-object_ids minus selection, skipping
model-hidden models so element-hide doesn't pile on top of model-hide),
and showAllElements (clears the override; model-hidden models stay
hidden, per the user's spec).

Bound in the View menu: H hide, Shift+H isolate, Alt+H show all.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.h

@@ -49,6 +49,7 @@ QT_END_NAMESPACE
 #include "OverlayRenderer.h"
 #include "Selection.h"
 #include "SidecarCache.h"
+#include "Visibility.h"
 
 // Matches GL_DRAW_INDIRECT_BUFFER layout for glMultiDrawElementsIndirect.
 struct DrawElementsIndirectCommand {
@@ -328,6 +329,26 @@ public:
     // or off-surface.
     std::unordered_set<uint32_t> picksInRect(const QRect& rect);
 
+    // Per-element visibility (independent of model-level hidden flag).
+    // External callers go through the convenience verbs below; the
+    // VisibilityState getter is exposed for read access (e.g. the host
+    // mirroring its state into a tree's grey-out style).
+    VisibilityState&       visibility()       { return visibility_; }
+    const VisibilityState& visibility() const { return visibility_; }
+    // Hide every currently-selected element.  Selection itself is
+    // preserved — toggling them back on with showAllElements() leaves
+    // the same items selected.
+    void hideSelectedElements();
+    // Hide every element NOT in the current selection (limited to
+    // visible models — model-hidden objects are left as-is so they
+    // stay model-hidden rather than picking up a redundant
+    // element-hidden flag too).  No-op when nothing is selected.
+    void isolateSelectedElements();
+    // Clear element-level visibility overrides for every loaded model.
+    // Model-level `hidden` flags are not touched — a hidden model stays
+    // hidden, matching the user's expectation of "show all *elements*".
+    void showAllElements();
+
     // Extended pick: returns the object id, world-space hit point, and
     // world-space surface normal at (x, y).  Renders the same pick pass as
     // pickObjectAt but reads back from two extra color attachments
@@ -729,6 +750,9 @@ private:
 
     // Selection — set + active id + per-object_id flags SSBO (binding=3).
     SelectionState selection_;
+    // Per-element visibility — CPU-only flag vector consulted by the
+    // cull, plus a canonical hidden-id set for mutation/reporting.
+    VisibilityState visibility_;
 
     // LMB-press state.  press_pick_id_ caches the object hit at press
     // time so the release path can apply click semantics without a