go: parse.sx — statements (return / short-decl / assign / block) + 9 tests [nothing]

First slice of Phase 2 statements. Replaces the func-decl ':body'
sentinel with real (:block STMTS) parsing.

gp-parse-stmt dispatches on the leading token:
  return [exprs]                — (list :return EXPRS)
  { ... }                       — nested block (recurses into block-body)
  lhs := exprs                  — (list :short-decl LHS-LIST EXPRS)
  lhs = exprs                   — (list :assign LHS-LIST EXPRS)
  lhs OP= expr                  — (list :assign-op OP LHS-LIST [EXPR])
  expr                          — bare expression statement
  var/const/type/func keywords  — fall through to gp-parse-decl

LHS may be a comma-separated list. Compound-assign covers all 11 Go
forms (+= -= *= /= %= &= |= ^= <<= >>= &^=).

gp-parse-block-body iterates: skips semis, terminates on '}', and for
non-trivial tokens calls gp-parse-stmt. **Two progress guards** added
to avoid infinite loops on unsupported syntax:

  * gp-block-body-loop force-advances one token if gp-parse-stmt
    returns nil without consuming.
  * gp-parse-composite-elems does the same when its expr parser
    returns nil — fixes a hang on '`if true {`x := 1`}`' where the
    parser was misreading `if true{...}` as a composite literal then
    spinning on `:=` inside the brace body.

Existing func/method decl tests updated from the ':body' sentinel to
the new (:block STMTS) shape. Old `gp-skip-block!` left as dead code
(removed once control-flow stmts make the misinterpretation issue
moot).

Control-flow stmts (if/for/switch/select/defer/go/break/continue) and
channel send (`ch <- v`) deferred to subsequent iterations.

parse 141/141, total 270/270.

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: 261 / 261 tests passing**
+**Total: 270 / 270 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
-| ✅ | parse | 132 | 132 |
+| ✅ | parse | 141 | 141 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |