W14: build C21 perform-mode harness + 5 pins (test-infra only)

The synchronous harness binds mocks as plain NativeFns, so no harness test
could exercise the real CEK perform/suspend/resume path — the HO+perform
element-drop class (S10) was structurally invisible (hosts.md C21).

Add harness-run-perform to spec/harness.sx: drives make-cek-state/
cek-step-loop, services each (perform {:op X :args L}) suspension from the
session's platform mocks (entry logged before invocation, C22-consistent),
cek-resumes with the mock value, loops to terminal; clear error on an
unmocked op. Shared arity dispatch extracted as harness-invoke-mock.

Pins (gate-C21-perform-mode-harness): single suspension, arithmetic-frame
resume, sequential performs, unmocked-op error, and the S10 probe — map
over a perform-suspending lambda keeps ALL 3 elements through 3
suspensions on the CEK path (localizing the drop class to serving-JIT).
290/0 under OCaml run_tests; harness self-suite green.

Caveat (documented): requires the runner's cek-* driver bindings — absent
on bare sx_server/MCP, the same runner-only-binding theme as section B.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

spec/harness.sx

@@ -13,6 +13,7 @@
     harness-set!
     make-interceptor
     install-interceptors
+    harness-run-perform
     io-calls
     io-call-count
     io-call-nth
@@ -76,6 +77,27 @@
         (session key value)
         (dict-set! (get session "state") key value)
         nil))
+    (define
+      harness-invoke-mock
+      :effects ()
+      (fn
+        (mock-fn args)
+        (if
+          (empty? args)
+          (mock-fn)
+          (if
+            (= 1 (len args))
+            (mock-fn (first args))
+            (if
+              (= 2 (len args))
+              (mock-fn (first args) (nth args 1))
+              (if
+                (= 3 (len args))
+                (mock-fn
+                  (first args)
+                  (nth args 1)
+                  (nth args 2))
+                (apply mock-fn args)))))))
     (define
       make-interceptor
       :effects ()
@@ -87,7 +109,7 @@
             ((entry {:op op-name :result nil :args args}) (log (get session "log")))
             (append! log entry)
             (let
-              ((result (if (empty? args) (mock-fn) (if (= 1 (len args)) (mock-fn (first args)) (if (= 2 (len args)) (mock-fn (first args) (nth args 1)) (if (= 3 (len args)) (mock-fn (first args) (nth args 1) (nth args 2)) (apply mock-fn args)))))))
+              ((result (harness-invoke-mock mock-fn args)))
               (dict-set! entry "result" result)
               result)))))
     (define
@@ -104,6 +126,14 @@
               (env-bind! env key interceptor)))
           (keys (get session "platform")))
         env))
+    (define
+      harness-run-perform
+      :effects ()
+      (fn
+        (session expr env)
+        (let
+          ((drive (fn (self state) (if (cek-suspended? state) (let ((req (cek-io-request state))) (let ((op (get req "op")) (args (or (get req "args") (list)))) (let ((mock-fn (get (get session "platform") op))) (when (nil? mock-fn) (error (str "harness-run-perform: no mock for op " op))) (let ((entry {:op op :result nil :args args}) (log (get session "log"))) (append! log entry) (let ((result (harness-invoke-mock mock-fn args))) (dict-set! entry "result" result) (self self (cek-resume state result))))))) (cek-value state)))))
+          (drive drive (cek-step-loop (make-cek-state expr env (list)))))))
     (define
       io-calls
       :effects ()