Merge commit from fork

Both fallbacks in splitPos relied on golang.org/x/text/search with
search.IgnoreCase, which performs Unicode equivalence matching far beyond
ASCII case folding. Combined with the validated-ASCII guarantee on every
SplitPath entry, that fallback turned non-PHP filenames into PHP scripts:

- when the inner loop hit a non-ASCII byte and the IndexString fallback
  returned -1, the loop broke without resetting match=false, so a stale
  match=true caused a non-existent .php to be reported (PoC:
  "/name.<U+00A1>.txt").
- search.IgnoreCase folded fullwidth, mathematical and circled letters
  onto ASCII, so "/shell.<math sans-serif php>",
  "/shell.<fullwidth p>hp", "/shell.<circled php>" were all detected as
  ".php" files.

Replace the fallback with strict byte-level ASCII case-insensitive
matching: any byte >= utf8.RuneSelf in the path can never be part of a
match, since SplitPath entries are validated ASCII-only and lower-cased
in Provision(). This keeps the hot path branch-light and removes the
x/text/search dependency from the main module.

Reported against FrankenPHP as GHSA-3g8v-8r37-cgjm and
GHSA-v4h7-cj44-8fc8. The vulnerable function in this module was adapted
from the same FrankenPHP code.