fix(simulation): pin SeedDomain::VoxelRelief discriminant + record T-1081 review notes

T-1081 review (Hoshe + Tyre): the seed_domain_discriminants_are_pinned test (the
D-224 guard that fails CI if a variant is renumbered — which would silently re-roll
every world's relief field) did not pin the new VoxelRelief = 11. Add the assertion.

Also fold Tyre's two Q-123 calibration notes into item 4: (i) sanity-check the
slope_q*3 + elev_q envelope on a synthetic high-slope body (both terms cap at 100);
(ii) the relief transect is a single diagonal slice (fine for isotropic fBm, revisit
if an anisotropic ridge field is added).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/seed.rs

@@ -274,6 +274,7 @@ mod tests {
         assert_eq!(SeedDomain::ChunkContext as u64, 8);
         assert_eq!(SeedDomain::Voxel as u64, 9);
         assert_eq!(SeedDomain::Cover as u64, 10);
+        assert_eq!(SeedDomain::VoxelRelief as u64, 11);
     }
 
     #[test]