wgpu streaming (5/4): per-chunk indices + LRU/distance eviction (stopgap)

Defers index buffers per-chunk (alongside vertex bytes) so streaming
fully delivers on its "don't load until visible" contract — the previous
per-model index buffer was upfront-loaded and tipped scenes >~1.5 GB into
allocator OOM at frame 1.

Adds residency tracking + a two-phase evictor: (1) drop LRU non-visible
chunks first, (2) if everything resident is visible-this-frame, drop the
farthest-from-eye chunk only when the candidate to load is closer. This
gives monotonic convergence to "closest visible chunks fit the budget"
instead of "first 4 win, rest never load."

Default budget set to 1 GB — explicitly a stopgap, documented inline.
The per-machine OOM ceiling on wgpu-native (caused by allocator
fragmentation from one VkDeviceMemory per createBuffer call) cannot be
solved by tuning this knob. The proper fix is a probed single-pool
buffer with sub-allocation, tracked under task #16.

Caveat: LOD1 indices are now force-disabled when chunking — per-chunk
buffers only carry LOD0. Re-enabling needs LOD1 to participate in the
chunk plan.

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

src/ifcviewer-wgpu-minimal/main.cpp

@@ -62,6 +62,10 @@ int main(int argc, char* argv[]) {
         "Enable streaming sidecar load. Reads metadata-only at load time; "
         "vertex chunks are deferred and loaded on demand as they become "
         "frustum-visible. Required for scenes that exceed GPU memory."});
+    parser.addOption({"streaming-vram-mb",
+        "Set the streaming residency budget in MB (default 1900). Tune "
+        "down to test browser-class memory ceilings; tune up if your GPU "
+        "+ driver are happy with larger allocations.", "mb"});
     parser.process(app);
 
     auto* viewport = new WgpuViewportWindow;
@@ -70,6 +74,16 @@ int main(int argc, char* argv[]) {
     if (parser.isSet("web-limits")) viewport->web_limits_       = true;
     if (parser.isSet("bvh"))        viewport->bvh_enabled_      = true;
     if (parser.isSet("streaming"))  viewport->streaming_enabled_ = true;
+    if (parser.isSet("streaming-vram-mb")) {
+        bool ok = false;
+        const uint64_t mb = parser.value("streaming-vram-mb").toULongLong(&ok);
+        if (ok && mb > 0) {
+            viewport->streaming_vram_budget_bytes_ = mb * 1024ull * 1024ull;
+        } else {
+            qWarning() << "--streaming-vram-mb: failed to parse"
+                       << parser.value("streaming-vram-mb");
+        }
+    }
 
     QWidget* container = QWidget::createWindowContainer(viewport);
     container->setMinimumSize(320, 240);