[k-kostin]

Summary

STL thumbnails currently require an installed 3D thumbnailer. Add a small CPU companion built with Folio's existing C++/Qt dependencies, so ordinary STL files can get a transparent thumbnail without starting F3D or a graphics context.

explicit user thumbnailer
  otherwise: bundled CPU STL helper
    if absent or declined: selected system thumbnailer

The helper runs outside the UI process. It reads a bounded snapshot, scans it twice without allocating a vertex graph, renders both sides with a depth buffer and 2×2 coverage samples, and declines malformed, colored or excessive geometry. CPU and system attempts share one deadline and existing process-session cleanup. Native failure identity includes helper availability/version, fallback and output bucket. Successful pictures use the existing shared cache.

Thanks to @thisisgm for the Flea renderer this camera, raster math and palette were adapted from. Its MIT notice is included.

Draft, stacked on #67 (7cbd76c6); rebase and repeat checks after that lands. This does not include the separate cancellation changes in #117. A local combined build passed the cancellation regressions, including the native STL route. Packaging companion: omacom/omarchy-pkgs#2704. Packagers must ship the companion and its additional license beside Folio; the pending install rules in #107 should include them when rebased.

Evidence

• Before: thumbnailsDrawStlWithoutAnExternalRenderer fails on the base with no external STL renderer.
  After: the real companion generates and caches the synthetic tetrahedron.
• Final bin/test: 220 Folio checks + 8 renderer checks passed, using isolated Qt image plugins matching the CI package set (the desktop also has KDE RAW plugins). Release build passed on x86_64.
• Renderer tests also passed under ASan/UBSan. A separate fixed-seed 4,000-input parser/raster exercise found no sanitizer errors. Checks cover binary/ASCII equivalence, winding/depth, premultiplied transparency, extreme coordinates, malformed input, output preservation and FIFO refusal.
• Provider tests cover user overrides, missing/broken helper, missing/invalid PNG, system fallback, failure-cache changes, one shared timeout and large-thumbnail refusal followed by a successful smaller request.

Local QML grid comparison: eight generated, non-personal STL fixtures, 12–960,000 triangles (largest 45.8 MiB), ten alternating pairs after warm-up. Empty application thumbnail cache, warm OS page cache; Ryzen 7 9700X, Qt 6.12, F3D 3.5 native plugin. Offscreen Qt Quick software backend; these are local measurements, not physical-screen frame latency or CI timings.

Metric	System F3D	Bundled CPU
First ready thumbnail, median / p95	562 / 605 ms	55 / 61 ms
All eight ready, median / p95	1004 / 1042 ms	139 / 150 ms
Whole-scope CPU time, median	3332 ms	276 ms
Whole-scope peak memory, median	880 MiB	123 MiB

Timing runs used the renderer commit 28bffb2; the final d87b9dd adds only bucket-aware failure identity and its regression. No user model or derived image is included.

Merge Danger

Door: two-way. Removing the companion restores the existing system-thumbnailer route.

Blast Radius: thumbnails. STL gets a new default monochrome renderer; explicit user overrides and colored-model fallback remain. Input is capped at 128 MiB / two million triangles, output at 1024 px, and raster work at 128 million bounding-box samples. No new runtime package or Rust toolchain is needed. aarch64 has not been built locally.