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.md

@@ -1,11 +1,11 @@
 # Go-on-SX Scoreboard
 
-**Total: 227 / 227 tests passing**
+**Total: 235 / 235 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
-| ✅ | parse | 98 | 98 |
+| ✅ | parse | 106 | 106 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |