CN: per-probe heterozygous BAF spread from the batch reference; --mosaic-prior/--switch-distance; Pi memory guard

scripts/pi_run.sh runs heavy jobs in their own process group and stops them when
MemAvailable drops below a floor (Raspberry Pi OS boots with cgroup_disable=memory, so a
systemd MemoryMax cap is not available); a 360-sample genotype run with cohort QC had
exhausted memory and the OOM killer took down the desktop session.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01814GKVzZVL3FuF3DeiZXhv

src/simurg/cn/model.py

@@ -71,3 +71,18 @@ def reference_stats(
         np.nan_to_num(het_c, nan=0.5).astype(np.float32),
         n_het,
     )
+
+
+def reference_het_spread(baf: npt.NDArray[np.float32]) -> npt.NDArray[np.float32]:
+    """Per-probe robust SD (1.4826·MAD) of heterozygous BAF (0.2 < BAF < 0.8) across reference samples; NaN when
+    fewer than 5 heterozygous samples."""
+    het = np.where((baf > 0.2) & (baf < 0.8), baf, np.nan)
+    with np.errstate(all="ignore"):
+        import warnings
+
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", RuntimeWarning)
+            med = np.nanmedian(het, axis=0)
+            mad = np.nanmedian(np.abs(het - med), axis=0)
+    n = np.isfinite(het).sum(axis=0)
+    return np.where(n >= 5, 1.4826 * mad, np.nan).astype(np.float32)