sx: step 8 — non-exhaustive match warnings

Emit a warning when a `match` expression on an ADT value misses one
or more constructors and lacks an `else`/`_` clause. Behaviour is
non-fatal — the match still runs, the warning goes to stderr.

- spec/evaluator.sx: helpers `match-clause-is-else?`, `match-clause-ctor-name`,
  `match-warn-non-exhaustive`, `match-check-exhaustiveness`. The latter
  reads the `*adt-registry*` (already populated by `define-type`),
  collects constructor patterns from clauses, and dedupes via an
  `*adt-warned*` env-bound dict so each (type, missing-set) warns once.
  Wired into `step-sf-match` via a `do` block before clause dispatch.

- hosts/javascript/platform.py: `host-warn` primitive (`console.warn`)
  + matching `hostWarn` js-id helper so the JS-transpiled spec code
  can call it directly. Spec code reaches JS via `sx_build target=js`.

- hosts/ocaml/lib/sx_runtime.ml + sx_primitives.ml: `host-warn` runtime
  helper (`prerr_endline`) and registered primitive.

- hosts/ocaml/lib/sx_ref.ml: HAND-PATCHED. `step_sf_match` now calls
  a hand-written `match_check_exhaustiveness` that handles both
  `AdtValue` and back-compat dict-shape ADT values. The OCaml side
  is *not* retranspiled because regenerating sx_ref.ml drops
  several preamble fixes (seq_to_list, string->symbol mangling,
  empty-dict literal bug). Future retranspile must reapply this patch.

- spec/tests/test-adt.sx: 5 new tests covering exhaustive,
  non-exhaustive (warning is non-fatal), `else` suppression,
  partial coverage with one missing constructor, and `_` wildcard
  suppression. Tests assert return values only — warnings go to
  stderr and are not captured.

Warning format: `[sx] match: non-exhaustive — TypeName: missing Ctor1, Ctor2`
Both hosts emit identical messages.

Tests: OCaml 4540 → 4545 (+5), JS 2586 → 2591 (+5). Zero regressions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

spec/tests/test-adt.sx

@@ -454,4 +454,33 @@
         (match
           (WX (LeftX 9))
           ((WX (LeftX v)) (str "left-" v))
-          ((WX (RightX v)) (str "right-" v)))))))
+          ((WX (RightX v)) (str "right-" v))))))
+  (deftest
+    "exhaustive match runs without error"
+    (do
+      (define-type ExA1 (CaA1 v) (CbA1))
+      (assert= 1 (match (CaA1 1) ((CaA1 x) x) ((CbA1) 0)))
+      (assert= 0 (match (CbA1) ((CaA1 x) x) ((CbA1) 0)))))
+  (deftest
+    "non-exhaustive match still returns value (warning is non-fatal)"
+    (do
+      (define-type ExA2 (CaA2 v) (CbA2))
+      (assert= 9 (match (CaA2 9) ((CaA2 x) x)))))
+  (deftest
+    "match with else clause suppresses non-exhaustive warning"
+    (do
+      (define-type ExA3 (CaA3 v) (CbA3) (CcA3))
+      (assert= "a" (match (CaA3 1) ((CaA3 x) "a") (else "other")))
+      (assert= "other" (match (CbA3) ((CaA3 x) "a") (else "other")))))
+  (deftest
+    "match with all-but-one constructor still runs"
+    (do
+      (define-type ExA4 (CaA4 v) (CbA4) (CcA4))
+      (assert= 5 (match (CaA4 5) ((CaA4 x) x) ((CbA4) 0)))
+      (assert= 0 (match (CbA4) ((CaA4 x) x) ((CbA4) 0)))))
+  (deftest
+    "match wildcard pattern suppresses non-exhaustive warning"
+    (do
+      (define-type ExA5 (CaA5 v) (CbA5))
+      (assert= 7 (match (CaA5 7) ((CaA5 x) x) (_ 0)))
+      (assert= 0 (match (CbA5) ((CaA5 x) x) (_ 0))))))