fix discarded-qualifiers warnings with GCC 16+

[graysky2]

strpbrk() returns char * even when passed a const char *, a known C standard wart. Explicitly cast the return value to char * at all nine call sites to satisfy -Werror=discarded-qualifiers

[bkuhls]

Duplicate of #156?

[graysky2]

Claude thinks this PR is a better approach:

• The POSIX signature of strpbrk is char *strpbrk(const char *s1, const char *s2) — it accepts a const char * input just fine, so casting the input in a.patch is unnecessary and misleading. It implies the function requires a mutable pointer, which it doesn't.
• Casting the return value (PR#158) is the idiomatic C workaround for the classic const-correctness quirk where strpbrk loses the const qualifier on its return. It's the same pattern used in the C standard library wrappers for strchr, strstr, etc.
• PR#156 casts on the input is effectively a no-op in terms of fixing the warning, and could confuse a future reader into thinking src's constness matters to strpbrk.