Design 3-A4i rung B (imagej-elphel-internal 2026-07-16 handoff), DP-readiness:
- gpu_pose_* / gpu_pose_candidate_* become FIXED physical slots; the CURRENT
  set is selected by a device-authoritative int flag.
- pose_lma_reduce_rms gains a final int* current_set arg and flips it on
  accept DEVICE-SIDE (NULL = no flip, standalone tests); rejection leaves it
  unchanged so the prior current raw pose remains the fallback.
- tp_proc_exec_pose_lma_resident_step: host std::swap removed; launch
  pointers come from cur_*/cand_* accessors; the host mirror syncs with a
  4-byte D2H after each step. exec_pose_fx_jacobian uploads/reads via the
  CURRENT set. The candidate-base DtoD invariant (ref rows pose-independent)
  is now documented at the copy site.
- test_pose_lma_step: pass + assert the no-flip side (not-accepted case).
C API unchanged. Gates: direct CUDA 5120/150 PASS (products 0.000244141,
candidate 9.69e-08, same as session-40); NVRTC resident accept-carryover +
worsened-rejection PASS (flip proven behaviorally); 19-float bit-exact PASS;
fx_jacobian PASS; compute-sanitizer(12.8) 0 errors; test_pose_corr_jna
bit-exact @tol 0.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>