Claude: refineMotionVectors() — multithreaded masking + pre-computed kernel

- Pre-compute integer-pixel raised-cosine mask kernel once before nseq loop
- Allocate fpixels_masked[nframes][width*height] once; clear per nseq with Arrays.fill
- Restructure masking: parallel outer loop over frames (nFr via AtomicInteger),
  inner loop over tiles — no write contention (non-overlapping tile guarantee)
- Pre-extract tile center/velocity into arrays before thread launch
- Replace new ImagePlus(FloatProcessor) debug calls with ShowDoubleFloatArrays.showArrays()
- Fix "centre" -> "center" in comments; naming: nFr for ai.getAndIncrement(), ntile for plain for