go: parse.sx — struct type expressions + 8 tests [proposes-ast]

Adds Go struct types to gp-parse-type:
  struct {}                       →  (list :ty-struct ())
  struct { x int }                →  (list :ty-struct [(:field [x] (:ty-name int))])
  struct { x int; y string }      →  multiple field rows
  struct { x, y int }             →  shared-type row (NAMES is a list)
  struct { inner struct { x int } }  →  nested struct types

gp-parse-struct-fields walks field rows tolerating ASI-inserted semis
(from newlines between fields). Each row collects 1+ names separated
by commas, then a single type that all the names share. Embedded
fields, field tags, and methods are deferred.

The :field shape (NAMES + TYPE) is a recurring multi-language pattern —
struct fields, func params, method receivers, var decls all map to it.
Logged in Blockers as a canonical-AST candidate
(ast-binding-group / ast-named-of-type); worth promoting once a second
consumer (parser of another statically-typed guest, or Go func decls)
exercises the same shape.

parse 98/98, total 227/227.

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: 219 / 219 tests passing**
+**Total: 227 / 227 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
-| ✅ | parse | 90 | 90 |
+| ✅ | parse | 98 | 98 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |