go: parse.sx — function calls + member access + 12 tests [consumes-ast proposes-ast]

Adds postfix expression forms per Go spec:
  f()  f(x)  f(x, y, z)       — function calls
  x.y  x.y.z  obj.method(x)   — selector / member access

gp-parse-postfix sits between gp-parse-unary and gp-parse-primary,
so calls and selectors bind tighter than any unary prefix — `-f(x)`
parses as `-(f(x))`, not `(-f)(x)`. Postfix is left-associative
(`x.y.z` = `(x.y).z`), so the loop iterates rather than recurses
on the LHS.

AST shapes:
  Call:     (ast-app FN ARGS)              — canonical
  Selector: (list :select OBJ "field")     — Go-specific tag

The selector shape is a kit gap — lib/guest/ast.sx ships ast-app but
no ast-select, despite `obj.field` being universal across Go, Rust,
Swift, TS, JS, Python, Ruby, Java, C#. Logged in Blockers; tagging
[proposes-ast]. Worth promoting on the next nominally-typed guest.

parse 49/49, total 178/178.

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

lib/go/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "language": "go",
-  "total_pass": 166,
-  "total": 166,
+  "total_pass": 178,
+  "total": 178,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
-    {"name":"parse","pass":37,"total":37,"status":"ok"},
+    {"name":"parse","pass":49,"total":49,"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"},