server: assert timeouts on the served http.Server, not just the constructor (closes #120)

The timeout tests called newHTTPServer directly, leaving Run's call
site unguarded: a Run that built its http.Server inline would drop
every timeout with the suite still green. TestRunWiresSocketTimeouts
now wires a Server as cmd/dnswatcher does (minus watcher and resolver,
so no live DNS), drives Run with an unbindable port so it stores its
http.Server and returns without listening, and asserts the served
server carries all four timeouts and both required relationships.
Reverting Run to an inline http.Server without timeouts fails it. The
near-tautological addr/handler test is dropped; the wiring test covers
the handler end to end.

The read-deadline note is corrected: a too-small ReadTimeout does not
make ReadHeaderTimeout unreachable (net/http applies it directly); it
installs an already-expired whole-request deadline. Verified against
the pinned go1.25 net/http.

Model: opus-4-8

internal/server/export_test.go

@@ -5,15 +5,20 @@ import (
 	"time"
 )
 
-// NewHTTPServer exports newHTTPServer for testing.
-func NewHTTPServer(
-	listenAddr string,
-	handler http.Handler,
-) *http.Server {
-	return newHTTPServer(listenAddr, handler)
-}
-
 // RequestTimeout exports the handler execution budget applied by
 // chimw.Timeout in SetupRoutes, so tests can assert the relationship
 // between it and the server's WriteTimeout.
 const RequestTimeout time.Duration = requestTimeout
+
+// SetListenPort overrides the port Run binds. A test uses it to hand
+// Run an unbindable port so ListenAndServe fails immediately and Run
+// returns after storing its http.Server.
+func SetListenPort(s *Server, port int) {
+	s.port = port
+}
+
+// HTTPServerOf returns the http.Server that Run built and stored, so a
+// test can inspect the timeouts the running server actually carries.
+func HTTPServerOf(s *Server) *http.Server {
+	return s.httpServer
+}