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>

plans/agent-briefings/sx-gate-loop.md

@@ -71,7 +71,11 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
       12-probe battery from the finding, errors compared by message)
 - [x] C22/K104 — FIXED harness (spec/harness.sx make-interceptor: log entry
       appended before the mock runs, :result updated via dict-set!) + 3 pins
-- [ ] C21 — real perform/suspend mode in harness
+- [x] C21 — BUILT `harness-run-perform` (spec/harness.sx): drives real CEK
+      suspend/resume, services performs from session mocks, C22-style
+      logging; 5 pins incl. the S10 map-over-perform probe (CEK keeps all
+      elements — the drop class is serving-JIT-side). Runner-only (needs
+      cek-* driver bindings)
 - [ ] C23 — adapter-dom render-output tests
 
 ### D. WASM corpus runner
@@ -87,6 +91,20 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 2026-07-04 — **C21 perform-mode harness (item C.3)**. Added
+  `harness-run-perform` to spec/harness.sx (exported): 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-resume`s with the
+  mock value, loops to terminal. Self-recursion via the `(self self …)`
+  pattern (avoids letrec-injection K06 territory). Extracted the arity
+  dispatch into shared `harness-invoke-mock`. 5 pins in
+  `gate-C21-perform-mode-harness` — notably the **S10 probe**: `(map (fn (u)
+  (perform …)) '("a" "b" "c"))` keeps ALL elements through 3 suspensions on
+  the CEK path, confirming the element-drop class is serving-JIT-side, not
+  CEK. Caveat noted in the docstring: needs the runner's cek-* driver
+  bindings (absent on bare sx_server/MCP — the env-parity theme again).
+  290/0. Test-infra-only.
 - 2026-07-04 — **C22/K104 throwing-mock fix + pins (item C.2)**. First
   actual FIX of the loop — in scope because spec/harness.sx is W14-owned
   test infrastructure (PLAN approach item 4 assigns "log IO before invoking