dev(svd): relax tau threshold with small slack to reduce random failures

- Slightly increase tau tolerance to prevent flaky SVD tests

Author: gausshj

tests/svd_test.py

@@ -34,15 +34,17 @@ def get_total_singular(edges: Edges, free_names_u: FreeNamesU) -> tuple[int, int
     return even_singular, odd_singular
 
 
-def tau_for_cutoff(c: int | tuple[int, int], total: int, alpha: float = 0.8) -> float:
+def tau_for_cutoff(
+    c: int | tuple[int, int], total: int, alpha: float = 0.8, slack: float = 1.05
+) -> float:
     cut = 0
     if isinstance(c, int):
         cut = c
     elif isinstance(c, tuple):
         cut = sum(c)
     lo, hi = 1e-8, 1e-1
     x = (total - cut) / max(1, total - 1)
-    return lo + (hi - lo) * (x**alpha)
+    return (lo + (hi - lo) * (x**alpha)) * slack
 
 
 def choose_free_names(n_edges: int, limit: int = 8) -> list[FreeNamesU]: