fix default extension files shadowed by same-name directories

[kisnner26]

when users.html and a users/ directory coexist, /users always redirects to the directory, making the extensionless page unreachable. try a readable sibling file with the configured default extension before handling an extensionless directory request.

/users/ continues to address the directory explicitly. if there is no matching sibling file, keep directory redirects and listings. carry the request method and query string into the file lookup.

fixes #629.

validation: npm test passes all 547 assertions across 43 suites on macos with node 26. regression coverage includes the collision, query strings, explicit directory access, child pages, missing sibling files, a custom extension and head. the collision cases fail before the fix. git diff --check passes.