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>

docs/moving_bodies.md

@@ -338,6 +338,31 @@ recorded as not runnable rather than replaced by another test.
   run with a moving body raises before upload/launch; stationary unaffected). No kernels, no GPU runs, no
   y-only VIV oscillator (S4), no force RHS.
 
+## 10a. S2a — fixed-centre moving links (implemented on `claude/moving-bodies-s2a`)
+
+- `MovingCircleLinks` (`kernels/step/moving_circle.cu`, dense double buffer)
+  runs before the static curved kernel when moving tables are attached: one
+  thread per (band cell, direction) slot, FP64 `{cx, cy, r}` at the applied
+  offset, q/hit/clamp/fallback in the order of `build_cut_links` with explicit
+  `_rn` operations (host/device bit parity is a test target, not a claim),
+  then the unchanged `apply_bouzidi_link` with spin wall velocity only.
+- `obs` keeps its contract (CME and torque from `apply_bouzidi_link`). A
+  separate per-body diagnostics buffer receives `cme`, the Wen term
+  `u_w * dm`, `link_dm` and the link count from the same link outputs; it is
+  zeroed with obs and only exists when moving circles do. Optional
+  `slot_debug` (q, fallback, rx, ry per slot) is null in production.
+- Displacement is disabled: `set_body(offset=..., vx=..., vy=...)` on a
+  translating circle raises before mutation, before and after initialize, so
+  no pending target can exist and `run`/`step`/direct `LBMStepper.step` all
+  hold at the applied geometry. `initialize()` never reads or consumes
+  `pending`. `stepper.blocked` is cleared only when the moving tables are
+  attached (initialize, stepper recreation).
+- S3 rule (not implemented): once targets are re-enabled, any pending target
+  must raise in `run`/`step` and in `LBMStepper.step` alike before any launch,
+  swap or step-count change; zero-step calls never consume it.
+- Not validation: a pass of the zero-motion gates says nothing about moving
+  geometry. GPU parity (G0a/G0b) is unverified until run on a free device.
+
 ## 11. Remaining decisions
 
 Structural RHS (section 6); motion-record layout; refill details; coupling