k256: debug_assert wNAF digit index stays in bounds

`wnaf_128` writes into a fixed 130-entry buffer; the bound holds for the
current `WNAF_WIDTH = 5` and the ≤128-bit GLV sub-scalars, but it's
implicit. Add a `debug_assert!` in the loop so that any future change to
`WNAF_WIDTH` that invalidates the bound is caught at test time rather
than silently writing out of bounds in worst-case inputs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 42Pupusas

k256/src/arithmetic/mul.rs

@@ -343,6 +343,7 @@ fn wnaf_128(k: &Scalar) -> [i8; WNAF_DIGITS] {
     let mut out = [0i8; WNAF_DIGITS];
     let mut i = 0;
     while (lo | hi) != 0 {
+        debug_assert!(i < WNAF_DIGITS);
         if (lo & 1) == 1 {
             // d = k mod 2^W, recentered into [-2^(W-1) + 1, 2^(W-1) - 1]
             let mut d = (lo & width_mask) as i64;