feat: const-ify Lu::det, Ldlt::det, inf_norm, det_errbound, and solve methods #86

Dependencies

• chore: bump MSRV to Rust 1.95 and adopt new stable features #67
• bug: inf_norm silently returns 0.0 for matrices containing NaN #85

Summary

Several public methods that are pure computation over [f64; D] / [[f64; D]; D] arrays could be const fn today but aren't. This would enable compile-time evaluation and is consistent with the existing const fn pattern established by det_direct, Vector::dot, etc.

Depends on: #67 (MSRV bump to 1.95, which stabilizes additional const-compatible float intrinsics) and #85 (fix inf_norm before const-ifying it)

Candidates

Easy (no trait methods, just loops and float ops)

• Lu::det — loop over diagonal with multiply-accumulate. Straightforward.
• Ldlt::det — same pattern as Lu::det.
• Matrix::inf_norm — needs manual loop replacing .iter().map().sum() (iterators aren't const-stable). Also depends on f64::abs() being const — check availability at MSRV 1.95.
• Matrix::det_errbound — uses f64::abs() and mul_add. Same abs() dependency.

Moderate (manual loops replacing iterator chains)

• Lu::solve_vec — the forward/back substitution loops are already manual; the iterator-based inner loops (enumerate().take(i), enumerate().skip(i+1)) would need conversion to while loops. The is_finite() check needs to be const-compatible.
• Ldlt::solve_vec — same pattern as Lu::solve_vec.

Blocked (needs generic_const_exprs or other unstable features)

• gauss_solve and exact methods — use BigInt/BigRational heap types, not const-eligible
• Augmented matrix types ([T; D+1]) — needs generic_const_exprs (v0.5.0 territory)

Approach

1. After chore: bump MSRV to Rust 1.95 and adopt new stable features #67 lands, audit which f64 methods (abs, is_finite, is_nan) are const-stable at MSRV 1.95
2. Convert eligible methods, replacing iterator chains with while loops where needed
3. Add const evaluation tests (similar to the existing det_direct_is_const_evaluable_* tests)

Benefits

• Compile-time evaluation of determinants, norms, and solves for known inputs
• Consistent API: if det_direct and Vector::dot are const, their peers should be too
• Foundation for v0.5.0's deeper const-generics work