feat: calibration helper scripts + DANIILIDIS solver default + dual-robot README

- scripts/: explore_and_calibrate.py (probe envelope from actual + random sampling),
  extend_calibration.py (loop fino a n_used >= TARGET_GOOD), repeatability.py
  (diagnostica bias vs noise sul marker-finder)
- config.yaml: calibration.method TSAI → DANIILIDIS (sweep su 49 sample H1 sx
  mostra 4× meno residuo di TSAI per via del bias intrinsics-dipendente di
  marker-finder)
- README: dual-robot G1/H1, scripts dir, caveat noti (wrist passthrough,
  marker-finder intrinsics floor, H1 4 DoF, /api/arms/go_neutral non proxato)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.yaml

@@ -40,7 +40,7 @@ grasp:
     - 0
 calibration:
   min_samples: 5
-  method: TSAI
+  method: DANIILIDIS  # sweep su 49 sample H1 sx: TSAI=62.5mm, DANIILIDIS=14.2mm trans residual
   file: data/calibration.yaml
   samples_file: data/samples.json
 chat: