The route map only answers paths that are known when the listener is
built. A route with an unbounded segment — /shop/<slug>, /order/<token>,
/posts/<id> — cannot be pre-registered: the token space is unbounded and
the product set changes while the server runs. Every such request became
a synthetic 404 that the application never got to see.
Add one optional member, called for anything `routes` missed, with the
full target still in request.path. Precedence is exact path, then the
query-stripped path, then fallback, then the 404 as before, so a
default-constructed listener behaves byte-identically and no call site
changes.
A hook rather than a pattern syntax: consumers that already have a
router — one shared between a wasm frontend and the server, so a URL
cannot mean different things to a crawler and to the app — keep using
it, and there is no second route table to disagree with the first.
PathWithoutQuery moves out of this file into :HTTP as an exported
PathWithoutQueryHTTP, since ListenerHTTP now needs the same split and a
fallback handler almost always does too.
ListenerAsyncHTTP1 starts accepting inside its constructor, so assigning
`listener.fallback` afterwards would race the accept loop; it gets a
constructor overload that installs the fallback before the thread
starts.
