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 <[email protected]>
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 <[email protected]>
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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

.github/workflows/ci.yml

@@ -4,6 +4,9 @@ on:
   pull_request:
   push:
     branches: [main]
+  schedule:
+    # Nightly: the heavy tests on main.
+    - cron: '23 3 * * *'
   workflow_dispatch:
 
 permissions:
@@ -65,8 +68,26 @@ jobs:
           components: rustfmt, clippy
           targets: wasm32-unknown-unknown
       - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+        with:
+          # Branches restore main's cache; saving their own would only evict it.
+          save-if: ${{ github.ref == 'refs/heads/main' }}
       - run: cargo xtask ci
 
+  heavy:
+    # The heavy verification and corpus tests (#[ignore]d) and the doc tests: on main, nightly
+    # and on demand, not on every pull request.
+    name: cargo xtask test --heavy
+    if: contains(fromJSON('["push", "schedule", "workflow_dispatch"]'), github.event_name)
+    runs-on: ubuntu-24.04
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
+      - run: cargo xtask test --heavy
+
   advisories:
     name: Dependency advisories and sources
     needs: changes