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>

hosts/javascript/platform.py

@@ -2124,6 +2124,13 @@ PLATFORM_JS_PRE = '''
   // hostError — throw a host-level error that propagates out of cekRun.
   function hostError(msg) { throw new Error(typeof msg === "string" ? msg : inspect(msg)); }
 
+  // hostWarn — emit a host-level warning to console (no-op if console missing).
+  function hostWarn(msg) {
+    var m = typeof msg === "string" ? msg : inspect(msg);
+    if (typeof console !== "undefined" && console.warn) console.warn(m);
+    return NIL;
+  }
+
   // Render dispatch — call the active adapter's render function.
   // Set by each adapter when loaded; defaults to identity (no rendering).
   var _renderExprFn = null;
@@ -4010,6 +4017,11 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False, has_deps=False, has_
   // -----------------------------------------------------------------------
   PRIMITIVES["error"] = function(msg) { throw new Error(msg); };
   PRIMITIVES["host-error"] = function(msg) { throw new Error(typeof msg === "string" ? msg : inspect(msg)); };
+  PRIMITIVES["host-warn"] = function(msg) {
+    var m = typeof msg === "string" ? msg : inspect(msg);
+    if (typeof console !== "undefined" && console.warn) console.warn(m);
+    return NIL;
+  };
   PRIMITIVES["try-catch"] = function(tryFn, catchFn) {
     try {
       return cekRun(continueWithCall(tryFn, [], makeEnv(), [], []));