fix(#396): an accessibility spec proved a setting survives navigating to the 404 page

[TortoiseWolfe]

The bug

colorblind-toggle.spec.ts navigated to /about to prove a colour-vision setting survives a page change. This app has never had an /about route.

So "navigate to another page" navigated to not-found.tsx, and the persistence the test claims to prove was only ever proven across the error page. It passed 7/7 either way, because the chrome it asserts on renders there too.

That's #396's "an E2E test that lands on the 404 route" class: nothing errors, the spec keeps running, and a green result means something weaker than it reads.

Pointed at /blog/, which exists. Still 7/7 against production — a strict improvement, not a repair of a broken test.

The guard is static, deliberately

#396 asks to "fail loudly when an E2E test lands on the 404 route." A runtime fixture wrapping every navigation would catch more — including computed paths like goto(`/blog/${slug}`) — but it changes behaviour for every spec on the required lane, and sweeping the whole suite found exactly one real instance.

I took the zero-risk half now rather than smuggling a lane-wide behaviour change in behind a one-line bug fix. The runtime half stays recorded in #396.

e2e-gotos-resolve.test.js enumerates the app router's real routes — handling (groups), [dynamic] and [...catchall] — and asserts every literal goto('/...') in the suite resolves to one.

Two allowlisted paths, each with a reason

path	why
/non-existent-page	the test is "404 page handles non-existent routes"
/__contrast-probe-unmatched-route__/	color-contrast.spec.ts's TEMPLATE_PROBES — a template has no URL of its own, so not-found.tsx must be reached deliberately (#425)

The test asserts each reason is longer than a token, so the allowlist can't rot into a place where real mistakes get parked.

Verification

Mutation-verified with the mutant confirmed present in the file first:

• restore the /about navigation → caught, naming colorblind-toggle.spec.ts:174 -> /about
• empty the allowlist → caught, proving the allowlist is load-bearing rather than decorative

test:scripts 424/424; type-check and lint clean.

Stated in the file so a green run isn't over-read: this cannot see computed paths, redirects that end on a 404, or routes that exist as files but fail to build.

Refs #396