W14: pin K18 expt-overflow float-promotion (test-only) + bootstrap gate briefing

The dc7aa709 quick-wins batch fixed `expt`'s silent 63-bit int wrap (now
promotes to float like +/*) but shipped no pinning test — a regression would
pass silently. Add spec/tests/test-gate-pins.sx suite gate-K18-expt-overflow
(4 tests, minimal reprs from plans/sx-review/core.md): small exponents exact,
2^62 and 2^100 do not wrap, 2^100 is a float. 4/4 green under OCaml run_tests.

Also bootstraps plans/agent-briefings/sx-gate-loop.md (the loop's own briefing,
absent until now) with the W14 checklist derived from PLAN.md §W14.

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

@@ -0,0 +1,88 @@
+# W14 — Test gate & conformance infrastructure loop
+
+Forge agent **ws-W14**. Role: build out **W14** from the SX review remediation plan
+(`plans/sx-review/PLAN.md`, §"W14. Test gate & conformance infrastructure") —
+*the enabler that makes every other fix verifiable*. One checklist item per fire.
+
+You are on branch `loops/sx-ws-w14`, worktree `/root/rose-ash-loops/sx-ws-w14`.
+
+## Hard guardrails (read every fire)
+
+- **TEST-ONLY.** No semantics edits. Do NOT touch `spec/evaluator.sx`,
+  `spec/primitives.sx`, `spec/parser.sx`, `spec/render.sx`, the OCaml kernel,
+  or any host runtime. W14 pins behavior with tests and productionizes the
+  *test/runner* surface; the actual fixes are other workstreams (W1–W12).
+  A pin that *fails* means the finding regressed — do NOT relax the assertion,
+  record it as a blocker.
+- **NO PUSH.** Commit locally on `loops/sx-ws-w14` only. Never push; never touch
+  `main` or `architecture`.
+- **`.sx` files: use `sx-tree` MCP tools only** (a hook blocks Read/Write/Edit
+  on `.sx`). `sx_write_file` takes params **`file`** and **`source`** (NOT
+  `content` — a wrong key yields a `yojson … got null` error and no write).
+  `.md`/`.sh`/`.ml` files: normal tools are fine.
+- **Never `pkill`/`kill` `sx_server`** — sibling loops share the binary. Bound
+  every run with `timeout` (e.g. `timeout 300 …`); if it hangs, let the timeout end it.
+- **One item per fire, then stop.** No batching.
+
+## Per-iteration procedure
+
+1. Pick the first unchecked `[ ]` in the checklist.
+2. Implement (test file or runner/harness change), lifting minimal repros from
+   the review lane files (`plans/sx-review/{core,hosts,conformance}.md`) — they
+   are a ready-made corpus of confirmed reprs.
+3. Build + run the affected tests:
+   `sx_build` (target ocaml) then
+   `timeout 300 ./hosts/ocaml/_build/default/bin/run_tests.exe <test-name>`
+   to run a single file. New `spec/tests/test-*.sx` files are auto-discovered.
+4. Confirm green (a pin must PASS on current HEAD — the fix already landed).
+5. Commit locally: `git add -A && git commit` with a `W14:` prefix.
+6. Tick the box, prepend one dated line to the Progress log, stop.
+
+## Checklist
+
+### A. Test-debt pins — dc7aa709's landed fixes shipped without regression tests
+Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
+`spec/tests/test-gate-pins.sx` (one `defsuite` per finding).
+
+- [x] K18 [W7] — `expt` overflow now float-promotes (no 63-bit wrap)
+- [ ] K20 [W7] — identify the landed W7 fix and pin it
+- [ ] K09/K11/K39 [W5] — landed special-form fixes, pin each
+- [ ] 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
+- [ ] S4 [conformance] — housekeeping repro, pin
+
+### B. Runner/production env unification
+- [ ] Audit runner-only bindings (`values`/`call-with-values` F7/K42, JS
+      fake sha3/equal?/apply/env-set! shims JS5) — inventory + failing pin
+      that a fresh `sx_server` reproduces the drift
+
+### C. Harness honesty
+- [ ] K19 — MCP `mcp_tree.ml` harness primitive table drift vs `sx_primitives`
+      (parity test)
+- [ ] C22/K104 — harness logs IO *before* invoking the mock (throwing-mock pin)
+- [ ] C21 — real perform/suspend mode in harness
+- [ ] C23 — adapter-dom render-output tests
+
+### D. WASM corpus runner
+- [ ] F2 — promote conformance's `run_wasm.js` prototype into CI
+
+### E. Epoch-loop protocol fuzz + skip-list
+- [ ] C3/C4/C5/C6/C7 — epoch protocol fuzz suite
+- [ ] F10 — hs-upstream skip-list so browser-only FAILs mean something
+- [ ] C9 — empty suite label
+
+### F. Differential battery
+- [ ] F8 — cross-host differential battery (same source, all hosts agree)
+
+## Progress log (newest first)
+
+- 2026-07-03 — **K18 expt-overflow pin (item A.1)**. Bootstrapped this briefing
+  from PLAN.md §W14 (the referenced file did not exist yet). Added
+  `spec/tests/test-gate-pins.sx` with suite `gate-K18-expt-overflow` (4 tests):
+  small exponents stay exact (`2^0=1`, `2^10=1024`), `2^62 > 0` (no negative
+  63-bit wrap), `2^100 > 0` (no wrap-to-zero), `2^100` is a number (float
+  promotion). Verified 4/4 green under the OCaml run_tests kernel. Test-only.
+
+## Blocked
+- (none)

spec/tests/test-gate-pins.sx

@@ -0,0 +1,33 @@
+;; ==========================================================================
+;; test-gate-pins.sx — W14 regression pins for dc7aa709's landed fixes
+;;
+;; The quick-wins batch (commit dc7aa709) landed real semantics fixes but
+;; shipped WITHOUT pinning tests, so a regression would pass silently. This
+;; file pins each confirmed-and-fixed finding with a minimal repro lifted
+;; from the review lane files (plans/sx-review/*.md). One suite per finding.
+;;
+;; TEST-ONLY: no semantics edits. If a pin fails, the fix regressed — do NOT
+;; relax the assertion; investigate the evaluator/primitive change.
+;; ==========================================================================
+
+;; --------------------------------------------------------------------------
+;; K18 [W7, high] expt silently wrapped at 63-bit int — now promotes to float
+;; like +/*. Repro (core.md): (expt 2 62) -> -4611686018427387904 (wrapped);
+;; (expt 2 100) -> 0. Fixed: both are positive floats.
+;; --------------------------------------------------------------------------
+(defsuite
+  "gate-K18-expt-overflow"
+  (deftest
+    "small integer exponents stay exact"
+    (do
+      (assert= (expt 2 0) 1)
+      (assert= (expt 2 10) 1024)))
+  (deftest
+    "expt 2^62 does not wrap to a negative int"
+    (assert (> (expt 2 62) 0)))
+  (deftest
+    "expt 2^100 does not wrap to zero"
+    (assert (> (expt 2 100) 0)))
+  (deftest
+    "expt 2^100 promotes to float"
+    (assert (number? (expt 2 100)))))