Raise bloom threshold to HDR 3.0; fix blit dst-mip layout warnings

The smooth brass case back of the pocket watch reflects large bright
sky regions of the Venice dawn environment at HDR values well above 1.
With the old threshold of 1.0 the entire metal surface passed the
bloom extract, and the blurred additive result washed it out to flat
white. Only genuinely hot highlights (HDR >= 3, near-white after ACES)
should bloom.

- Default bloomThreshold 1.0 -> 3.0; Stats slider range extended to 8
  with a logarithmic scale
- Texture mip generation: transition the whole mip chain to
  TRANSFER_DST up front; blitting into never-transitioned destination
  mips triggered one validation warning per level

App/editor/StatsPanel.cpp

@@ -95,7 +95,8 @@ void StatsPanel::Draw(const EditorContext& ctx)
         if (bloom)
         {
             f32 threshold = renderer.GetBloomThreshold();
-            if (ImGui::SliderFloat("Threshold", &threshold, 0.0f, 2.0f))
+            if (ImGui::SliderFloat("Threshold", &threshold, 0.0f, 8.0f,
+                                   "%.1f", ImGuiSliderFlags_Logarithmic))
             {
                 renderer.SetBloomThreshold(threshold);
             }