Fix: Canonical redirects hijacking matched routes (#13)

[Levdbas]

Problem

When a mapped route's slug happened to match the slug of a media attachment (e.g. an image), WordPress would redirect the request straight to the raw upload file instead of rendering the route.

Root cause: WordPress's redirect_canonical() runs on template_redirect and issues a 301 to the raw attachment file whenever the resolved main query is_attachment() is true — this is expected behavior since attachment pages are disabled by default (wp_attachment_pages_enabled). The problem is that this redirect fires (and exits) before the template_include filter that Routes::load() registers ever runs, so a matched route gets silently overridden.

Fix

Routes::load() now cancels WordPress's canonical redirect via the documented redirect_canonical filter (add_filter('redirect_canonical', '__return_false')) whenever it successfully sets up a template. Once a route has matched and is rendering a response, it should always take full control — WordPress's own canonical/attachment redirect logic should no longer have a say.

This only applies when a route actually resolves to a template via Routes::load(); requests that don't match any route, or that match a route which never calls Routes::load(), are completely unaffected.

Tests

• testRouteSurvivesAttachmentRedirect — reproduces the original bug: creates a colliding attachment, maps a route to the same slug, forces the main query to resolve as that attachment, and asserts the response is 200 (not a 301 to the raw file). Confirmed this test fails without the fix.
• testUnmatchedRouteDoesNotAffectCanonicalRedirects — sanity check that regular WordPress canonical redirects (e.g. legacy ?p=123 links) still work when no route matches at all.
• testMatchedRouteWithoutLoadDoesNotAffectCanonicalRedirects — sanity check that a route matching and running its callback without calling Routes::load() doesn't affect canonical redirects either.

All 25 tests pass, verified stable across multiple random test orderings.

Files changed

• Routes.php — the fix
• tests/RoutesTest.php — regression + sanity tests

Fixes #13