lbm: fixed-centre moving-circle cut links (S2a)

MovingCircleLinks computes the cut links of each translating circle from an
FP64 {cx, cy, r} table over its swept band (same q/hit/clamp/fallback order
as build_cut_links, explicit _rn operations) and applies the unchanged
apply_bouzidi_link with spin wall velocity. obs keeps its contract; a
separate per-body diagnostics buffer records CME, the Wen term u_w*dm,
link_dm and the link count. Moving tables are built at initialize and
attached to the stepper, which then launches the kernel before the static
curved kernel; until then direct and run steps stay blocked. Displacement
is disabled: set_body offset/vx/vy on a translating circle raises before
mutation. Contract section 10a documents S2a and the S3 pending rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_state_regressions.py

@@ -297,7 +297,7 @@ def test_passive_overlap_failure_keeps_live_state(device, entry, second_x):
                           stream=SimpleNamespace(synchronize=device.synchronize),
                           stepper=SimpleNamespace(blocked=None))
     for name in ("_require_usable", "_moving_bodies", "_reject_moving_bodies",
-                 "_validate_moving_bodies"):
+                 "_validate_moving_bodies", "_attach_moving"):
         setattr(sim, name, getattr(Simulation, name).__get__(sim))
     with pytest.raises(ValueError, match="active fluid receivers|unique cut-link writers"):
         if entry == "sync":