fix(kpm): use combined absolute+relative tolerance in M6-2 dual-mode tests

The fixed 1e-5 absolute tolerance in solve_linear_system_2x2_matches_cpp
and solve_symmetric_linear_system_3x3_matches_cpp is too tight for f32
values with magnitude > ~10. Apple Silicon ARM64 uses FMA differently
from x86_64, producing tiny precision differences that exceed 1e-5 for
larger results.

Example failure on macOS ARM64:
    rust=-40.826614, cpp=-40.8266, diff=1.5e-5

The diff of 1.5e-5 represents a relative error of ~3.7e-7, which is
well within f32 precision but exceeds the absolute tolerance.

Fix: use combined tolerance:
    tol = max(1e-5, |value| * 1e-6)

This gives:
- |x| <= 10: tolerance unchanged at 1e-5 (preserves existing strictness)
- |x|  = 100: tolerance 1e-4 (~1e-6 relative)
- |x|  = 1000: tolerance 1e-3 (~1e-6 relative)

These bounds are consistent with f32's ~7-digit precision and
accommodate platform-specific FMA rounding without losing test
sensitivity for typical values.

Surfaced now because PR #117's new dual-mode CI step exposed this
pre-existing issue on macOS runners. The math itself is correct;
only the test tolerance was too strict.

Refs #116

Author: kalwalt

crates/core/src/kpm/freak/math.rs

@@ -2112,13 +2112,20 @@ mod dual_mode_tests {
                     if diff > max_diff {
                         max_diff = diff;
                     }
+                    // Combined absolute + relative tolerance: f32 has ~7
+                    // decimal digits, so values around magnitude M need
+                    // tolerance ~ M * 1e-6. The 1e-5 floor handles values
+                    // near zero. This accommodates platform-specific FMA
+                    // rounding differences (Apple Silicon vs x86_64).
+                    let tol = 1e-5_f32.max(x_rust[i].abs() * 1e-6);
                     assert!(
-                        diff < 1e-5,
-                        "solve_linear_system_2x2 diverged at x[{}]: rust={}, cpp={}, diff={}",
+                        diff < tol,
+                        "solve_linear_system_2x2 diverged at x[{}]: rust={}, cpp={}, diff={}, tol={}",
                         i,
                         x_rust[i],
                         x_cpp[i],
-                        diff
+                        diff,
+                        tol
                     );
                 }
                 compared += 1;
@@ -2183,13 +2190,16 @@ mod dual_mode_tests {
                     if diff > max_diff {
                         max_diff = diff;
                     }
+                    // See solve_linear_system_2x2_matches_cpp for tolerance rationale.
+                    let tol = 1e-5_f32.max(x_rust[i].abs() * 1e-6);
                     assert!(
-                        diff < 1e-5,
-                        "solve_symmetric_linear_system_3x3 diverged at x[{}]: rust={}, cpp={}, diff={}",
+                        diff < tol,
+                        "solve_symmetric_linear_system_3x3 diverged at x[{}]: rust={}, cpp={}, diff={}, tol={}",
                         i,
                         x_rust[i],
                         x_cpp[i],
-                        diff
+                        diff,
+                        tol
                     );
                 }
                 compared += 1;