go: parse.sx — switch + select + 8 tests; stmts done [shapes-scheduler]

Adds Go's switch and select statements:

  switch TAG { case V1, V2: a; case V3: b; default: c }
  switch { case cond: ... }                            — tagless
  select { case x := <-ch: a; case ch <- v: b; default: c }

AST shapes:
  (list :switch TAG CASES)             — TAG nil for tagless
  (list :case VALUES BODY)             — VALUES is expr-list
  (list :select CASES)
  (list :select-case COMM-STMT BODY)   — COMM-STMT is send/recv-assign/bare-recv
  (list :default BODY)

gp-parse-case-body reads stmts until the next case/default/}/eof
without consuming the terminator — used by both switch and select.

select-case parsing reuses gp-parse-stmt for the comm-stmt, so all
four shapes (send, x := <-ch, x = <-ch, bare <-ch) fall out from the
existing stmt parser. Composite-lit suppression is engaged for the
switch tag expression.

Type-switch (`switch v := x.(type) { case int: ... }`) is the one
deferred shape; needs the `.(type)` pseudo-syntax recognised in the
expression layer. Phase 2 statement coverage is otherwise complete.

This is also a chiselling iteration for scheduler sister kit. Diary
updated with select-case design insights:

  * All four select-case shapes share (list :select-case STMT BODY)
    — kit primitive sched-select accepts a uniform list of cases.
  * Default vs no-default determines blocking semantics. Erlang's
    `receive ... after Timeout -> ...` is the analogue — both fit
    "non-blocking fallback case" in the kit API.

parse 169/169, total 298/298.

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

lib/go/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "language": "go",
-  "total_pass": 290,
-  "total": 290,
+  "total_pass": 298,
+  "total": 298,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
-    {"name":"parse","pass":161,"total":161,"status":"ok"},
+    {"name":"parse","pass":169,"total":169,"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"},