go: parse.sx — package/import/var/const/type declarations + 10 tests [consumes-ast]

First slice of Phase 2 declarations:
  package main                      →  (list :package "main")
  import "fmt"                      →  (ast-import "fmt")    [from kit]
  var x int                         →  var-decl + :field binding
  var x = 5                         →  init only (type inferred)
  var x int = 5                     →  both type and init
  var x, y int = 1, 2               →  multi-name shared type
  const Pi = 3.14                   →  const-decl
  const C int = 42                  →  typed const
  type T int                        →  named alias
  type Point struct { x, y int }    →  named struct

New gp-parse-top dispatches on the leading keyword: routes
package/import/var/const/type to gp-parse-decl; everything else
still goes through gp-parse-expr. Existing expression tests are
unaffected (cur won't be a decl keyword at expression start).

var/const decls use the (:field NAMES TYPE) shape from the
ast-binding-group proposal — first concrete cross-deliverable use:
struct fields, var decls, const decls all envelope through the
same node. That's the smell test for whether the kit shape is
right; so far it's clean.

import uses the canonical ast-import from lib/guest/ast.sx — first
direct use of a kit constructor for a declaration shape.

Grouped/parenthesized decls (var (...), import (...), const (...),
type (...)) and func decls (with method receivers + named params)
deferred to subsequent iterations.

parse 124/124, total 253/253.

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: 243 / 243 tests passing**
+**Total: 253 / 253 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
-| ✅ | parse | 114 | 114 |
+| ✅ | parse | 124 | 124 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |