wgpu: probed-size pool replaces per-chunk createBuffer

Drops the per-machine "guess the OOM ceiling" budget knob in favour of
a single buffer pool whose capacity is *probed* at device-init time.
The runtime answers the question: descend from min(maxBufferSize, 4 GB)
through OOM error scopes, accept the largest size that allocates
cleanly. On a desktop wgpu-native v29 box this lands at 2 GB; on
browser-class platforms it'll land at 256 MB – 1 GB depending on the
implementation. Same code path either way.

Architecture:
- WgpuBufferPool (new): single WGPUBuffer + free-list sub-allocator
  with adjacent-range coalescing and first-fit. 256 B alignment for
  storage-binding offsets.
- Chunks now hold (pool_vertex_offset, pool_vertex_size) and
  (pool_index_offset, pool_index_size) instead of per-chunk WGPUBuffer
  handles. Load = pool.alloc + queueWriteBuffer. Unload = pool.free.
- Bind groups bind pool_.buffer() at the chunk's specific (offset, size)
  for both the vertex and index storage bindings.
- Eviction queries pool.largest_free_run_bytes() instead of a tracked
  budget; the two-phase LRU/distance evictor's policy is unchanged.

What this fixes:
- No more gpu-alloc-rs fragmentation OOM: one VkDeviceMemory block
  instead of N per-chunk blocks with rounding overhead. On the test
  dataset (~3 GB on disk, 562 k visible instances) the wgpu backend
  now runs through to render without OOM at any point.
- No --streaming-vram-mb knob, no hardcoded budget constant, no
  per-machine calibration. The pool size adapts to whatever the
  runtime grants.

Notes:
- Error scope probing: wgpu-native v29 classifies "Not enough memory
  left" as WGPUErrorType_Validation, not OutOfMemory. We push both
  filters (nested) and treat either firing as probe failure.
- The 4 GB probe cap is principled, not magic: above that, wgpu-native's
  advertised maxBufferSize is sometimes a sentinel (1 TB) that just
  forces wasteful halving steps. 4 GB is the largest buffer any
  realistic WebGPU implementation will grant a single allocation today.
- Pool destroy()/release happens after model release in shutdown() so
  the underlying buffer outlives every bind group that references it.

Follow-ups: spatial chunking (task #22) for finer eviction granularity;
cull perf needs work at 100+ models / 1M+ instances (separate from
streaming concerns).

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

src/ifcviewer-wgpu-minimal/main.cpp

@@ -62,10 +62,6 @@ 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;
@@ -74,16 +70,6 @@ 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);