Reserve a pressure condition on Jet, velocity remains the only implemented one

Jet gains a last field condition='velocity'. 'pressure' raises
NotImplementedError in __post_init__ before profile/amplitude validation, so a
pressure jet never reaches surface_link_velocity, tables or kernels; unknown
values raise ValueError. Amplitude stays a lattice velocity (not a flow rate);
no pressure units, field reuse, tables or kernel changes are introduced.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit fbf6f1d122)

tests/unit/test_jet_condition.py

@@ -0,0 +1,32 @@
+"""Jet boundary-condition hook: velocity implemented, pressure reserved (host only, no CUDA)."""
+import pytest
+
+from CelerisLab.body.jets import Jet, Parallel, Tube
+
+
+def region():
+    return Tube((16., 12.), 0., 3., .7)
+
+
+def test_default_is_velocity_and_survives_amplitude_change():
+    jet = Jet(region(), Parallel(0.), amplitude=.01)
+    assert jet.condition == "velocity"
+    assert jet.with_amplitude(-.02).condition == "velocity"
+    # positional calls keep their meaning; condition is the new last field
+    assert (Jet(region(), Parallel(0.), "parabolic", .01, "velocity")
+            == Jet(region(), Parallel(0.), "parabolic", .01))
+
+
+def test_pressure_is_rejected_before_amplitude_validation():
+    # An amplitude that the velocity check would reject must not decide the error.
+    for amplitude in (0., 5., float("nan")):
+        with pytest.raises(NotImplementedError, match="pressure jets are reserved"):
+            Jet(region(), Parallel(0.), amplitude=amplitude, condition="pressure")
+    with pytest.raises(NotImplementedError):
+        Jet(region(), None, profile="no-such-profile", condition="pressure")
+
+
+def test_unknown_condition_is_a_value_error():
+    for condition in ("Velocity", "flow_rate", "", None):
+        with pytest.raises(ValueError, match="condition"):
+            Jet(region(), Parallel(0.), amplitude=.01, condition=condition)