go: parse.sx — index x[i] + slice x[a:b]/x[a:b:c] + 12 tests [proposes-ast]

Adds the bracket postfix branch:
  a[0] / a[i] / a[i+1] / m["key"]             → (list :index OBJ IDX)
  a[:] / a[1:] / a[:2] / a[1:2] / a[1:2:3]    → (list :slice OBJ LOW HIGH MAX)

LOW/HIGH/MAX are AST nodes or nil for omitted indices. The 4th MAX
slot is only populated by the three-index full-slice form.

Two new lib/guest/ast.sx kit gaps surfaced (logged in plans/go-on-sx.md
Blockers):

  * No :index node — universal across guests with arrays/maps.
  * No :slice node — Python/Rust/Swift/JS/Ruby all need at minimum the
    two-index form. Go's three-index variant is more specialised but
    fits in the same shape with an optional fourth slot.

Parser is permissive on a[1::3] (strict Go rejects, but the type phase
can enforce the grammar; lexer/parser stays loose).

Chained (a[0][1]) and mixed-with-selector (a[0].field) cases work via
the existing left-associative postfix loop.

parse 61/61, total 190/190.

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

lib/go/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "language": "go",
-  "total_pass": 178,
-  "total": 178,
+  "total_pass": 190,
+  "total": 190,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
-    {"name":"parse","pass":49,"total":49,"status":"ok"},
+    {"name":"parse","pass":61,"total":61,"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"},