ifcviewer: promote runtime perf knobs, drop always-on settings

Promotes five env-var-driven knobs to AppSettings + the settings dialog
(min pixel radius, motion min pixel radius, LOD1 pixel threshold, HiZ
resolution, HiZ on/off).  Defaults: motion min pixel radius is now 10
(was 0/disabled) and IFC_HIZ_MOTION is on by default — the strict
view-projection gate reverts via env var =0 when chasing HiZ
correctness bugs.  ViewportWindow connects each *Changed signal so
changes invalidate cached cull state and take effect on the next
frame.

Removes "Load Property Data Source" and "Apply Coordinate Operation"
from the settings dialog: both are now hardcoded on.  The basic-info
property fallback (used when there's no live IFC source for an object,
e.g. .ifcview without a sibling) now triggers organically when
ElementRegistry::findEntity returns null instead of being gated on a
user toggle.  Federation::guessFederatedFalseOrigin lost its
apply_coordinate_operation parameter and now uses
georef.has_coordinate_operation directly.

src/ifcviewer/settings.rst documents the remaining diagnostic env vars
(IFC_HIZ_MOTION, IFC_CULL_THREADS, IFC_SKIP_MDI, IFC_MAX_SUBDRAWS,
IFC_FPS_HITCH_MS, IFC_SUBDRAW_DIAG, IFC_LOD_*) plus a cross-walk from
the old promoted-knob env-var names to their new QSettings keys.

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

src/ifcviewer/SceneLoader.cpp

@@ -241,7 +241,7 @@ void SceneLoader::applySidecarData(uint32_t mid, SidecarData data) {
     // Restore the cached CoordinateOperation into the model so
     // modelGeoref(mid) returns it without needing the IFC source.  Prevents
     // sidecar-loaded models from silently losing their georef when the
-    // .ifc/.rdb sibling is absent or AppSettings.loadDataSource is off.
+    // .ifc/.rdb sibling is absent.
     {
         ModelGeoref& gr = model.georef;
         gr.has_coordinate_operation = data.has_coordinate_operation != 0;
@@ -276,8 +276,6 @@ void SceneLoader::applySidecarData(uint32_t mid, SidecarData data) {
 }
 
 void SceneLoader::startDataSourceLoad(uint32_t mid) {
-    if (!AppSettings::instance().loadDataSource()) return;
-
     auto it = models_.find(mid);
     if (it == models_.end()) return;
 
@@ -355,13 +353,6 @@ void SceneLoader::onStreamerFinished() {
 
             qint64 ms = it->second.load_timer.elapsed();
             emit loadedFromStream(mid, ms);
-
-            // Slot(s) above run synchronously (sidecar write uses element_map_,
-            // not ifcFile()); drop the parsed file now to save memory if the
-            // user has opted out of keeping a property data source.
-            if (!AppSettings::instance().loadDataSource()) {
-                it->second.streamer->setIfcFile(nullptr);
-            }
         }
     }