go: types.sx — declaration checking (var/const/type + :=) + 12 tests [nothing]

Phase 3 cont. Adds go-check-decl which dispatches on AST shape and
returns either the extended context or a :type-error:

  :var-decl     (:field NAMES TYPE-or-nil) EXPRS-or-nil
  :const-decl   (same shape; same logic in v0 — mutability later)
  :short-decl   LHS-LIST EXPRS         (lhs is a list of :var nodes)
  :type-decl    NAME TYPE              (type alias)

New helpers:

  go-default-type      — untyped-int → int, untyped-float → float64,
                         etc. Used when inferring var x = EXPR.
  go-check-exprs-against — every expr assignable to the declared type.
  go-bind-names-to-synth  — pair names with default-typed synth of
                            corresponding exprs; extends ctx.

The canonical Go pitfall flows through end-to-end now:

  (go-check-decl ctx (go-parse "var x float64 = 42 / 7"))
  →  ctx + (x → float64)

Because: 42/7 synthesises to ty-untyped-int (binop result of two
untyped operands), then go-check-exprs-against uses go-type-assignable?
to check ty-untyped-int → ty-name "float64" — :ok via the
untyped-int-to-any-numeric assignability rule. The 6 (integer) result
gets float-converted on assignment, never floated mid-computation.

types 40/40, total 345/345.

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

lib/go/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "language": "go",
-  "total_pass": 333,
-  "total": 333,
+  "total_pass": 345,
+  "total": 345,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
-    {"name":"types","pass":28,"total":28,"status":"ok"},
+    {"name":"types","pass":40,"total":40,"status":"ok"},
     {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},