ifcviewer: budget the geometry cache and make required allocations fallible

Loading enough models drove the chunk pool to the driver's refusal point,
after which the first click aborted: the pick attachments are allocated
lazily, wgpu-native reported their OOM as a validation error nobody
observed, and the invalid views reached wgpuQueueSubmit, which panics
across the FFI boundary. Two policy defects compounding: the cache was
allowed to take the last byte, and nothing but the pool's own growth was
treated as fallible.

GPU memory is now two tiers. Required allocations (per-pixel attachments,
a model's metadata buffers, readback staging) are eager, deterministic
and fallible; the chunk pool is an elastic cache that grows only to a
budget and yields whenever a required allocation fails.

- GpuBudget (pure, unit-tested): desktop derives the budget from the
  driver's free-memory report minus a reserve for the attachments at 4K;
  web keeps the wasm-heap cap; either lowers it on pressure. The budget's
  source differs per platform, the mechanism does not.
- GpuAllocScope: the OOM/Validation error-scope dance in one place,
  synchronous on wgpu-native, provisional on Dawn-web. BufferPool's
  inline copy now uses it.
- BufferPool::shrinkToCapacity releases whole sub-buffers newest-first
  after the owner empties them; growth clamps to the budget instead of
  overshooting.
- ViewportCore::allocateRequired runs any required creation under a
  scope and, on failure, lowers the budget, evicts and releases cache
  sub-buffers, waits for the device to reclaim them, and retries until
  it fits or the cache is at its floor. Pick attachments are created with
  the other attachments in configureSurface; render() skips a frame
  rather than submit invalid views; a model whose buffers cannot fit is
  not loaded instead of aborting.

Verified on a 4 GB GeForce: the pool clamps itself at the derived budget
(256+256+67 MB for a 579 MB budget) and, in a standalone check against
the real device, a pool grown to the driver's refusal point observes a
failed required allocation, releases 320 MB and succeeds on retry.

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

src/bonsaiviewer/MainWindow.cpp

@@ -566,7 +566,11 @@ void MainWindow::setupLoader() {
                 .arg(stats.total_triangles)
                 .arg(stats.gl_draw_calls)
                 .arg(double(stats.vram_used_bytes) * mb, 0, 'f', 0)
-                .arg(double(stats.vram_capacity_bytes) * mb, 0, 'f', 0);
+                // Used against what the cache may grow to; the pool's
+                // momentary capacity only until the budget is known.
+                .arg(double(stats.vram_budget_bytes > 0
+                                ? stats.vram_budget_bytes
+                                : stats.vram_capacity_bytes) * mb, 0, 'f', 0);
         // Device total is only known when a driver backend answered.
         if (stats.device_vram_total_bytes > 0) {
             text += QString(" | Device %1/%2 MB")