fix: prevent viewport export clipping at high resolutions

[jacobvanbeets]

Summary

Fixes the viewport export clipping the splat model along a curved edge at higher resolutions (worse the higher you go), which only happens on the first export from a new viewpoint — re-exporting from the same view produces a correct image.

Root cause

The viewport export renders once and reads the image back immediately. The VkSplat renderer sizes its per-frame scratch (visible-primitive and tile-instance capacity) from deferred, one-frame-late high-water marks. The first render at a new viewpoint/resolution can exceed those marks and render capacity-clamped — the depth/tile-ordered tail of primitives/tile-instances is dropped — which is the curved clip that grows with resolution (tile-instance count scales with resolution). The interactive viewport hides this because it self-heals on the next frame, but a single-shot export saves the clamped frame. Re-exporting from the same view works because the deferred readback has since grown the marks.

Fix

Drive that self-heal synchronously for one-shot captures instead of capturing the first frame:

• Add VksplatViewportRenderer::previewCaptureSettled(), set from the start-of-frame deferred poll. It reports that the previously rendered frame produced complete, unclamped content using the steady-state macro chain (rejecting the legacy macro warm-up frame and partial-stats frames).
• In RenderingManager::renderPreviewImageToPreviewSlotWithState, re-render the Preview slot until previewCaptureSettled() is true (bounded by kMaxPreviewSettlePasses = 8), then read back. A pass >= 1 guard ties the signal to the current view, which keeps the tiled (very-high-res) path correct since each tile is a different sub-view.
• Settling is scoped to the export capture overloads (renderPreviewImageRgb8/Rgba8 + tiled path). Film-strip thumbnails (FloatRgb), asset previews (model overloads), and depth captures keep their single-render behavior, so there is no perf regression there.

Typical convergence is 2–4 passes; the cap only guards a pathological non-converging case.

Files changed

• src/visualizer/rendering/vksplat_viewport_renderer.hpp / .cpp
• src/visualizer/rendering/rendering_manager.hpp
• src/visualizer/rendering/rendering_manager_viewport.cpp

Testing

• Load a large model and export at 4K / 8K / 16K / 32K from a freshly moved viewpoint; the first export is now complete (no curved clip / black regions) and matches a re-export from the same view.
• vksplat.render.visible_clamped / vksplat.render.macro_instances_clamped no longer fire for the frame that is saved.
• Transparent PNG export and the tiled (>~264M px, e.g. 32K) path verified.

Closes #1304