CI: shard tests across 4 runners; heavy tests on main and nightly (#350)

* CI: build tests once and run them in 4 nextest shards

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

* CI: quote the shard arguments

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

* CI: extract the test archive into the checkout; skip the heaviest tests on pull requests

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

* Docs: CI shards and the tests pull requests skip

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

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

CONTRIBUTING.md

@@ -153,6 +153,19 @@ in the PR. `cargo xtask test` builds the tests with it and then runs every test
 `cargo xtask test -- shell`. Tests run beside each other on a busy machine, so a test must not
 depend on wall-clock timing.
 
+**CI shards the tests.** One job builds them into a cargo-nextest archive and four jobs run a
+quarter each, so every test runs in its own process: don't rely on state left by another test.
+Pull requests skip a few heavy tests (one to four minutes each), which run on every push to
+`main` and nightly; check the `main` run after merging. They are listed in
+`.config/nextest.toml` and the [testing page](docs/book/src/dev/testing.md):
+
+- `solvecraft-engine`: `tests::hostile_params_never_panic`, everything in `tests::robustness`,
+  `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`
+- `solvecraft-kernel`: `tests::boolean_identity_on_tangent_primitives`
+- `solvecraft-ui-egui` scenarios: `acceptance_sensor_enclosure`
+
 **The reference tests** are 76 parts in [`tests/oracle/`](tests/oracle/), each a `recipe.json`
 in our own format with an `expected.json` beside it. SolidCraft rebuilds the recipe and compares
 body count, volume, area, centre of mass and bounding box (and, where there is a `reference.stl`,