Robustness goldens: one test per seed, so CI spreads them (full runs ~33 → ~15-20 min) (#1851)

Thanks @jyviko.

CONTRIBUTING.md

@@ -180,7 +180,7 @@ Pull requests skip a few heavy tests (one to four minutes each), which run on ev
 `.config/nextest.toml` and the [testing page](docs/book/src/dev/testing.md):
 
 - `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),
+  the seeds of `tests::robustness_goldens` (`seed_00` … `seed_95`; pull requests that touch `crates/kernel` run every 8th),
   `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`, the `atomic_commands` contract tests