fix: loosen low_mach FP-stability threshold to 2e-7

The low_mach test exercises HLLC's low-Mach correction at near-zero
Mach, where (u_R - u_L) is fundamentally small. Verrou MCA sampling
variance puts the max_dev distribution typically around 5e-8 with a
tail occasionally hitting ~1e-7, making the previous 1e-7 threshold
flaky. Loosen to 2e-7 to absorb sampling noise.

After #1408 (WENO5 sum-of-squares beta) merges, the WENO contribution
to the cancellation chain disappears on uniform grids and the threshold
can be tightened back.

Author: sbryngelson

toolchain/mfc/fp_stability.py

@@ -156,8 +156,8 @@ def _get_source_context(fname: str, lineno: int, context: int = 2) -> str:
         "name": "low_mach",
         "description": "1-D water shock with low_Mach=1 HLLC correction active",
         "compare": ["cons.1.00.000050.dat", "prim.3.00.000050.dat"],
-        "threshold": 1e-7,
-        "ill_cond": "low_Mach correction: velocity perturbation ~u/c cancels severely at M≈0",
+        "threshold": 2e-7,
+        "ill_cond": "low_Mach correction: velocity perturbation ~u/c cancels severely at M≈0 (threshold loosened to 2e-7 to absorb MCA sampling variance)",
     },
 ]