Exercise 3: 405 With Allow vs. a Real 404 — Possible Solution
====================================================================

THE SETUP
------------------------------
    router = Router()
    router.add_route(
        'POST', '/subscribe',
        lambda **p: 'subscribed', name='subscribe',
    )

    status, handler, params, allowed = router.resolve('GET', '/subscribe')
    print(status, sorted(allowed) if allowed else allowed)
    # 405 ['POST']

    status, handler, params, allowed = router.resolve('GET', '/totally-unregistered-path')
    print(status, allowed)
    # 404 None

WHY THE FIRST CASE DESERVES A 405, NOT A 404
------------------------------
/subscribe is a real, registered path - Router.resolve() finds a
route whose regex matches it and adds 'POST' to its own allowed set
before ever checking the method. The resource genuinely exists at
that path; the client just tried to reach it the wrong way. HTTP's
own 405 status exists specifically for this - "the resource exists,
try again with a different method" - and the Allow header this
chapter's own app() builds from allowed tells the client exactly
which method(s) would have worked, without it having to guess.

WHY THE SECOND CASE DESERVES A PLAIN 404
------------------------------
No route's pattern ever matches /totally-unregistered-path under any
method at all, so allowed stays an empty set and resolve() returns
'404', None instead. There is nothing true to say about which methods
"would have worked" here, because nothing about this path is
registered in the first place - a 405 with an empty or fabricated
Allow header would be actively misleading, claiming a resource
exists when it doesn't.

WHY THIS WORKS AS AN ANSWER
------------------------------
It verifies both branches of resolve()'s own real behavior on two
genuinely different real inputs rather than asserting the distinction
abstractly, and the explanation ties each status code back to what
allowed actually contains in that case - non-empty means "found the
path, wrong method," empty means "never found the path at all."