feat: std.http handler carries an opaque ctx word (PLAN-HTTPZ A1 prep)

Server.init(cfg, handler, ctx); the handler signature gains a usize
third argument delivered verbatim per dispatch — typically a pointer
to the app's own state, since the server owns the call site. A bare
(req, resp) handler had no way to reach app state without globals.
examples/1633 pins the round trip.

examples/1633-http-server.sx

@@ -7,9 +7,11 @@
 
 PORT :: 18933;
 
-handler :: (req: *http.Request, resp: *http.Response) {
+handler :: (req: *http.Request, resp: *http.Response, ctx: usize) {
     if req.path == "/hello" {
         resp.body = concat("hello ", req.method);
+        // the ctx word arrives verbatim (init passed 77)
+        if ctx != 77 { resp.status = 500; resp.body = "ctx lost"; }
         return;
     }
     if req.path == "/echo" {
@@ -105,7 +107,7 @@ main :: () -> i32 {
         request_count = 3,
         max_conn = 8,
     };
-    srv, se := http.Server.init(cfg, handler);
+    srv, se := http.Server.init(cfg, handler, 77);
     if se { print("server init failed\n"); return 1; }
 
     buf : [4096]u8 = ---;