feat(http1): dispatch unmatched paths to an optional fallback handler

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.

interfaces/Crafter.Network-HTTP.cppm

@@ -29,6 +29,15 @@ namespace Crafter {
         std::string body;
     };
 
+    // Everything in a request target up to '?'. Both listeners dispatch on
+    // this when the full target is not a registered route, so `/thing?x=1`
+    // reaches the handler registered for `/thing`: browsers append query
+    // strings freely and registering every variant is not a workable API.
+    // Exported because a fallback handler generally needs the same split.
+    export inline std::string_view PathWithoutQueryHTTP(std::string_view target) {
+        return target.substr(0, target.find('?'));
+    }
+
     export inline HTTPRequest CreateRequestHTTP(std::string method, std::string path, std::string authority) {
         HTTPRequest r;
         r.method = std::move(method);