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.

library/modules/std/http.sx

@@ -145,9 +145,13 @@ Server :: struct {
     lfd: i32 = -1;
     conns: [*]Conn = null;
     own_alloc: Allocator;
-    handler: (*Request, *Response) -> void;
+    // The handler's third argument is `ctx`, an opaque word the app
+    // gave init — typically a pointer to its own state (store handle,
+    // config), since the server owns the call site.
+    handler: (*Request, *Response, usize) -> void;
+    ctx: usize = 0;
 
-    init :: (cfg: Config, handler: (*Request, *Response) -> void) -> (Server, !HttpErr) {
+    init :: (cfg: Config, handler: (*Request, *Response, usize) -> void, ctx: usize) -> (Server, !HttpErr) {
         lfd := socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0);
         if lfd < 0 { raise error.Bind; }
         one : i32 = 1;
@@ -175,7 +179,7 @@ Server :: struct {
         }
         return Server.{
             cfg = cfg, loop = lp, lfd = lfd, conns = slots,
-            own_alloc = oa, handler = handler,
+            own_alloc = oa, handler = handler, ctx = ctx,
         };
     }
 
@@ -401,7 +405,7 @@ Server :: struct {
         // be parsed as a dot-call on a function named `handler`)
         h := self.handler;
         resp : Response = .{};
-        h(@req, @resp);
+        h(@req, @resp, self.ctx);
 
         c.served += 1;
         keep := req.keep_alive and c.served < self.cfg.request_count;