CLAUDE: CUAS-RT overhaul - real-base synthetic-grid injection, level/time ROIs, 3d3 bypass, DNN offset/stride/window

Reworks the synthetic B-measurement and detector flow on the real pyramid base:

- Load: the real -CUAS-MERGED-CUAS stack is always the pyramid base (full-length
  levels); when curt_synth_src, the synthetic grid is loaded separately (scaled,
  NOT LoG'd) and injected TILED into each selected pyramid level AFTER the pure
  pyramid is built - same grid at every level, real noise averaged (sqrt(2)/level,
  printed). Both file picks use newestFile(). The old load-time mixing and the
  curt_synth_bg_avg (N=) decimation are retired.
- Level selection unified on curt_c5_levels for ALL heavy paths (3d3/conv5d,
  C5P-direct, DNN); cheap temporalAverage pyramid still builds up to the max
  selected level. Pyramid-depth guard stops building when a level would have <1
  frame (was NegativeArraySizeException on short sequences).
- Timing ROI curt_time_from/to (timestamp seconds; last-4-of-seconds shortcut,
  rollover-aware, 3-decimal fixed display) clips saved RECT/HYPER/DNN/C5P stacks
  and gates the DNN inference (compute-window, not just save).
- curt_3d3_en (default false) bypasses the 3d3 coarse-velocity path and its
  ~W*H*9 arrays. Full-velocities hyperstack render is guarded by a free-heap
  estimate (it is also an unfilled stub) and skipped if it would not fit.
- DNN: curt_dnn_stride (1 testing / 4 production 50%-overlap), and -DNN-OFFSET
  visualization (dx, dy, s over the ROI) from the offset channels.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>