test: seeded goldens for the robustness seeds (#826)

tests/robustness/goldens.json records what each of seeds 0-95 builds: its steps, how it ended (ok, or the kind and the start of the error it was refused with) and, for the design it left, the bodies' total volume and area (within a relative 1e-6) and their face, edge and shell counts. tests::robustness_goldens fails on any change; ROBUSTNESS_GOLDENS_BLESS="why" records one, with the reason and the seeds it moved. Pull requests that touch crates/kernel run every 8th seed; pushes to main and the nightly run every seed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/dev/testing.md

@@ -76,7 +76,8 @@ On GitHub, one job builds the tests into a
 Pull requests skip a few heavy tests (each takes one to four minutes); they run on every push to
 `main`, nightly, and in `cargo xtask test`:
 
-- `solidcraft-engine`: `tests::hostile_params_never_panic`, everything in `tests::robustness`,
+- `solidcraft-engine`: `tests::hostile_params_never_panic`, everything in `tests::robustness` and
+  `tests::robustness_goldens` (pull requests that touch `crates/kernel` run its `sample_*` part),
   `pattern_suppress_tests::enclosure_arc_extinction_keeps_a_real_sharp_inner_lip_after_resize_and_reopen`,
   `pattern_suppress_tests::enclosure_datums_follow_wall_height_and_board_depth_with_material_proofs`,
   `cmd::plastic::tests::enclosure_snap_fit_follows_resized_front_wall`