go: parse.sx — interface type expressions + 8 tests; type expressions DONE [nothing]

Adds Go interface type expressions:
  interface {}                              →  empty
  interface { Close() }                     →  no-param method
  interface { String() string }             →  with single return
  interface { Read([]byte) (int, error) }   →  multi-return method
  interface { Stringer }                    →  embedded named iface
  interface { io.Reader }                   →  qualified embedded
  interface { io.Reader; Close() error }    →  mixed

gp-parse-interface-elems walks elements tolerating ASI semis. Each
element is either:
  (list :method NAME PARAMS RESULTS)
  (list :embed TYPE)

Method params/results reuse gp-parse-func-type-params/results — the
shape is identical to a free-standing func type. Go 1.18+ type sets
(interface { ~int | ~float64 }) are deferred until the generics
sub-deliverable.

With this, the full Phase 2 **type expressions** sub-deliverable is
complete (pending only field tags, struct/iface embeds details,
variadic, named func params, generics — all flagged later).

parse 106/106, total 235/235.

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

lib/go/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "language": "go",
-  "total_pass": 227,
-  "total": 227,
+  "total_pass": 235,
+  "total": 235,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
-    {"name":"parse","pass":98,"total":98,"status":"ok"},
+    {"name":"parse","pass":106,"total":106,"status":"ok"},
     {"name":"types","pass":0,"total":0,"status":"pending"},
     {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},