go: types.sx — func-decl + stmt-level dispatch + 7 tests [nothing]

Phase 3 cont. Adds:

  * go-check-func-decl — binds the function in the outer ctx (recursive
    self-reference will work once call-checking lands), extends the
    body's ctx with each :field param group via go-ctx-extend-field
    (the binding-group shape's *third* consumer in the type checker;
    five total across parser+typer when counted with struct fields,
    var-decls, const-decls, func params, method receivers).
  * go-check-stmt — dispatches on :return / :assign / :var-decl /
    :const-decl / :short-decl / :type-decl / :block; falls back to
    go-synth for expression statements.
  * go-check-block — threads ctx through stmts so that decls inside
    the block extend the ctx for subsequent stmts.
  * go-check-return-list — each return expr assignable to the
    corresponding declared result type; mismatch counts are typed.
  * go-check-assign / go-check-assign-pairs — RHS assignable to LHS
    synthesised type, count mismatch typed.
  * Helpers: go-decl-params-to-ty-list (flattens :field NAMES TYPE to
    a flat list of N types), go-extend-with-params (folds extend-field
    over a param-group list), go-repeat-ty.

Coverage tests:
  func empty() {}                                          → ok
  func add(x, y int) int { return x + y }                  → ok
  func bad() int { return "hi" }                           → typed error
  func sig(x int) int                                      → signature-only binds
  func sumsq(x, y int) int { return x*x + y*y }            → params visible
  func two() int { var x int = 1; var y int = 2;           → nested decl
                   return x + y }
  func g() int { var x int; x = 5; return x }              → assign verified

types 47/47, total 352/352.

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

lib/go/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "language": "go",
-  "total_pass": 345,
-  "total": 345,
+  "total_pass": 352,
+  "total": 352,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
-    {"name":"types","pass":40,"total":40,"status":"ok"},
+    {"name":"types","pass":47,"total":47,"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"},