W14: pin K09/K11/K39 landed W5 special-form fixes (test-only)

Three dc7aa709 fixes shipped without pinning tests:
- K09: R7RS longhand (unquote-splicing X) now splices (was silent zero-splice)
- K11: guard re-raise sentinel gensym'd — a user value shaped like
  (list '__guard-reraise__ X) is data, not a forged re-raise
- K39: (do ((fn (x) x) 5) 99) -> 99, not a misparsed Scheme do-loop

Add suites gate-K09-longhand-unquote-splicing, gate-K11-guard-reraise-forgeable,
gate-K39-do-iife-head to spec/tests/test-gate-pins.sx with exact reprs from
plans/sx-review/core.md. 261 passed / 0 failed under OCaml run_tests.

Test-only: no semantics edits, no push.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -46,7 +46,7 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 - [x] K18 [W7] — `expt` overflow now float-promotes (no 63-bit wrap)
 - [x] K20 [W7] — `contains?` now supports dict key membership
-- [ ] K09/K11/K39 [W5] — landed special-form fixes, pin each
+- [x] K09/K11/K39 [W5] — longhand `unquote-splicing`, guard sentinel gensym, `do` IIFE-head
 - [ ] K49 [W8] — render depth/cycle guard (infinite recursive component)
 - [ ] crit-2 [W1] — signal-return frame key (verify the pin is non-vacuous)
 - [ ] C1/C1b [W3] — HTTP-mode concurrency fixes, pin
@@ -77,6 +77,16 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 2026-07-03 — **K09/K11/K39 W5 special-form pins (item A.3)**. Three suites
+  added to `spec/tests/test-gate-pins.sx`: `gate-K09-longhand-unquote-splicing`
+  (R7RS longhand `(unquote-splicing X)` now splices, incl. empty-list case;
+  shorthand still works), `gate-K11-guard-reraise-forgeable` (a body/clause
+  value shaped like `(list '__guard-reraise__ X)` is returned as data, not
+  misread as a re-raise — sentinel is now gensym'd), `gate-K39-do-iife-head`
+  (`(do ((fn (x) x) 5) 99)` → 99, not a misparsed do-loop — exact core.md
+  repro). Gotchas hit and fixed: quasiquoted bare idents are *symbols* not
+  strings, and `assert=` compares with `=` (not `equal?`, which returns false
+  on these spliced lists). 261 passed / 0 failed under OCaml run_tests. Test-only.
 - 2026-07-03 — **K20 contains?-dict pin (item A.2)**. Mapped K-codes by
   core.md severity order (K17 append!, K18 expt, K19 harness-drift, K20
   contains?-dict). Added suite `gate-K20-contains-dict` to

spec/tests/test-gate-pins.sx

@@ -9,6 +9,7 @@
 ;;
 ;; TEST-ONLY: no semantics edits. If a pin fails, the fix regressed — do NOT
 ;; relax the assertion; investigate the evaluator/primitive change.
+;; NB: assert= uses `=` (not `equal?`); compare lists with `=`.
 ;; ==========================================================================
 
 ;; --------------------------------------------------------------------------
@@ -56,3 +57,65 @@
   (deftest
     "contains? still works on string substrings"
     (assert (contains? "hello" "ell"))))
+
+;; --------------------------------------------------------------------------
+;; K09 [W5, high] R7RS longhand (unquote-splicing X) silently no-spliced —
+;; only shorthand ,@/`splice-unquote` was recognized, so the longhand
+;; serialized literally (zero-splice). Fixed: aliased to splice-unquote.
+;; Repro (core.md).
+;; --------------------------------------------------------------------------
+(defsuite
+  "gate-K09-longhand-unquote-splicing"
+  (deftest
+    "longhand unquote-splicing splices a list"
+    (assert=
+      (quasiquote
+        (1
+          (unquote-splicing (list 2 3))
+          4))
+      (list 1 2 3 4)))
+  (deftest
+    "longhand unquote-splicing of an empty list contributes nothing"
+    (assert=
+      (quasiquote (0 (unquote-splicing (list)) 9))
+      (list 0 9)))
+  (deftest
+    "shorthand splice-unquote still works"
+    (assert=
+      (quasiquote (a (splice-unquote (list 2 3)) z))
+      (list (quote a) 2 3 (quote z)))))
+
+;; --------------------------------------------------------------------------
+;; K11 [W5, high] guard re-raise sentinel was a plain forgeable symbol — a
+;; body/clause legitimately returning (list '__guard-reraise__ X) was
+;; misread as a re-raise of X. Fixed: sentinel gensym'd per execution, so a
+;; user value with that head is returned as data. Repro (core.md).
+;; --------------------------------------------------------------------------
+(defsuite
+  "gate-K11-guard-reraise-forgeable"
+  (deftest
+    "body value shaped like the sentinel is returned as data"
+    (assert=
+      (guard (e (true "caught")) (list (quote __guard-reraise__) "hi"))
+      (list (quote __guard-reraise__) "hi")))
+  (deftest
+    "clause returning the forged sentinel is not re-raised"
+    (assert=
+      (guard
+        (e (true (list (quote __guard-reraise__) "forged")))
+        (error "boom"))
+      (list (quote __guard-reraise__) "forged"))))
+
+;; --------------------------------------------------------------------------
+;; K39 [W5, med] `do` misparsed a first form whose head is a list (an IIFE)
+;; as a Scheme do-loop binding spec. Repro (core.md): (do ((fn (x) x) 5) 99)
+;; threw "first: expected list, got 5"; expected 99. Fixed: `do` is begin.
+;; --------------------------------------------------------------------------
+(defsuite
+  "gate-K39-do-iife-head"
+  (deftest
+    "do with an IIFE first form returns the last form (not a do-loop)"
+    (assert= (do ((fn (x) x) 5) 99) 99))
+  (deftest
+    "do with a single IIFE form returns its value"
+    (assert= (do ((fn () 42))) 42)))