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>

plans/lib-guest-scheduler.md

@@ -231,6 +231,24 @@ real result.
 
 _Newest first. Append one dated entry per milestone landed._
 
+- 2026-05-27 — Follow-up from same Phase 2 work: **`select` AST shape**
+  landed. Each case is `(list :select-case COMM-STMT BODY)` where
+  COMM-STMT is one of `:send`, `:short-decl` (recv into new var),
+  `:assign` (recv into existing var), or a bare receive expression
+  `(:app (:var "<-") [chan])`. The shape is uniform across all four
+  comm-stmt kinds — the kit's `sched-select` primitive should accept a
+  list of cases each described by `(direction chan value-target?)` and
+  let the kit's runtime pick a ready case. That uniformity is what
+  makes a single kit primitive cover all four Go case shapes.
+
+  Also: Go's `select` with `default` makes the multiplexer non-blocking;
+  without default it blocks until a case is ready. The kit primitive
+  should mirror this — present-or-absent default determines blocking
+  semantics. Erlang's `receive ... after Timeout -> ...` is a similar
+  pattern with a timeout case rather than default; the kit primitive
+  should handle both as instances of "non-blocking-fallback case."
+  Source: Go-on-SX commit `parse.sx — switch + select`.
+
 - 2026-05-27 — From Go-on-SX Phase 2 (parser side, ahead of scheduler
   implementation): the **parsed AST shapes** for Go's concurrency
   primitives have landed and are worth recording before Phase 5 builds