test(nxz): restore 100% coverage on memlimit.ts after parser refactor (#121)

The PR #120 extract-method refactor introduced three defensive paths
in parseSuffixed that vitest's v8 instrumentation flagged as
unreachable: the '?? ""' fallbacks on regex capture groups
(unreachable when exec returns non-null) and the 'mult === undefined'
guard (unreachable because the regex constrains the suffix to keys
present in the multiplier map).

- Replace 'match[1] ?? ""' and 'match[2] ?? ""' with 'as string'
  type assertions: compile-time only, no runtime branch for v8 to
  instrument, and not flagged by biome's noNonNullAssertion rule
  (unlike non-null '!' which would re-introduce the lint warning we
  resolved in PR #117 fix-round 1).
- Wrap the noUncheckedIndexedAccess-required mult-undefined guard
  with /* v8 ignore start ... stop */: TypeScript strict mode demands
  the check but the regex+map alignment makes it structurally
  unreachable. Project convention is start/stop pairs (never 'next').

Coverage: lines 95.2 -> 100, branches 83.3 -> 100 on memlimit.ts.
707 tests still pass byte-identical.

Author: oorabona

packages/nxz/src/memlimit.ts

@@ -40,10 +40,12 @@ function parseSuffixed(
 ): bigint | null {
   const match = regex.exec(s);
   if (!match) return null;
-  const num = match[1] ?? '';
-  const suffix = (match[2] ?? '').toUpperCase();
+  const num = match[1] as string;
+  const suffix = (match[2] as string).toUpperCase();
   const mult = multipliers[suffix];
+  /* v8 ignore start: defensive fail-closed; regex + map keys are pre-aligned, branch unreachable */
   if (mult === undefined) return null;
+  /* v8 ignore stop */
   return BigInt(num) * mult;
 }