Fast PR CI: small hostile-test caps, heavy tests in a separate job (#43)

* test: hostile-params test runs under small session work caps

Session gains Limits, the defaults of mesh.tet.generate's max_steiner and
max_tets (production values unchanged; a parameter still overrides them;
file.open keeps them). The registry-wide hostile-params test runs on
sessions with small caps, so the needle-triangle tessellation it meshes is
refused at once instead of meshing to the 50k Steiner cap: the engine test
binary drops from ~350 s to ~20 s locally.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* ci: move heavy tests to a heavy job on main, nightly and on demand

Four slow tests are #[ignore = "heavy: ..."] (every corpus recipe, the
cantilever convergence study, the explicit-vs-modal cantilever period, the
generic-predicate rational cross-check); fast representative tests of each
area stay in the PR gate. cargo xtask test --heavy runs every ignored test
(benchmarks included) and the doc tests; cargo xtask test and ci run the
rest without the doc tests. The new heavy CI job runs it on pushes to main,
nightly and on workflow_dispatch. Branch jobs restore main's rust cache and
no longer save their own. AGENTS.md documents the rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* test: the three slow LBM verification cases run in the heavy job

The Stokes-drag sphere array, the staircase-pipe convergence study and the
developing-channel case are heavy (21 s, 11 s, 6 s alone); plane
Poiseuille, Couette, Taylor-Green, reproducibility and the stability
limits stay in the PR gate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/lbm/tests/verification.rs

@@ -191,6 +191,7 @@ fn pipe(k: usize) -> (f64, f64) {
 }
 
 #[test]
+#[ignore = "heavy: cargo xtask test --heavy (CI heavy job)"]
 fn pipe_poiseuille_on_a_staircase_wall_converges() {
     let runs: Vec<(f64, f64)> = [4, 8, 16].iter().map(|&k| pipe(k)).collect();
     let l2: Vec<f64> = runs.iter().map(|r| r.0).collect();
@@ -261,6 +262,7 @@ fn sphere_drag(r: f64, l: usize, steps: u64) -> (f64, f64) {
 }
 
 #[test]
+#[ignore = "heavy: cargo xtask test --heavy (CI heavy job)"]
 fn stokes_drag_of_a_sphere_array_matches_hasimoto() {
     let coarse = sphere_drag(3.0, 16, 1500);
     let fine = sphere_drag(6.0, 32, 3000);
@@ -272,6 +274,7 @@ fn stokes_drag_of_a_sphere_array_matches_hasimoto() {
 }
 
 #[test]
+#[ignore = "heavy: cargo xtask test --heavy (CI heavy job)"]
 fn channel_with_a_parabolic_inlet_and_a_pressure_outlet_develops_poiseuille_flow() {
     // Plates 1 apart, 6 long; inlet peak 0.1; outlet at gauge pressure 0.
     let (h, l, nu, umax, n) = (1.0, 3.0, 0.05, 0.1, 16);