go: parse.sx — go/defer/send/for-range + 9 tests [shapes-scheduler]

Adds Go's concurrency + iteration primitives to the statement parser:

  go EXPR                     →  (list :go EXPR)
  defer EXPR                  →  (list :defer EXPR)
  ch <- v                     →  (list :send CHAN VALUE)
  for range COLL { ... }      →  (list :range-for nil nil nil COLL BODY)
  for k := range C { ... }    →  (list :range-for :short-decl KEY nil COLL BODY)
  for k, v := range C { }     →  (list :range-for :short-decl KEY VAL COLL BODY)
  for k, v = range C { ... }  →  (list :range-for :assign KEY VAL COLL BODY)

gp-for-find-range pre-scans the for-header (to '{' or eof) looking
for the 'range' keyword; if present, dispatches to gp-parse-for-range
which handles the four range shapes. C-style and while-like and
infinite are now in gp-parse-for-c-style — gp-parse-for is just a
dispatcher.

Send statement detection lives in the LHS-list branch of gp-parse-stmt:
after parsing a single LHS expression, '<-' triggers (:send LHS RHS).
Channel-recv (`<-ch`) was already parsed as unary `<-` in the expression
layer, so both directions cover.

This is the **chiselling-relevant iteration** for the scheduler sister
kit: the AST shapes Go-on-SX will eventually feed into the kit's
scheduler primitives (sched-spawn, sched-defer, chan-op) have landed.
Sister-plan diary updated with three design insights:

  * :go / :defer both wrap a single expr — kit's sched-spawn should
    accept a thunk uniformly across Erlang's spawn(M,F,A) and Go's
    go fn().
  * :send carries CHAN+VALUE symmetrically with the unary <- recv —
    both reduce to (chan-op direction chan value) in the kit.
  * `for v := range ch` uses the same :range-for shape as range-over-
    slice; the scheduler kit's range dispatch is where chan-recv ⇄
    iteration polymorphism lives.

parse 161/161, total 290/290.

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

plans/lib-guest-scheduler.md

@@ -231,5 +231,37 @@ real result.
 
 _Newest first. Append one dated entry per milestone landed._
 
+- 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
+  the scheduler.
+
+  ```
+  go EXPR              → (list :go EXPR)
+  defer EXPR           → (list :defer EXPR)
+  ch <- v              → (list :send CHAN VALUE)
+  <-ch                 → (list :app (:var "<-") [CHAN])   ; unary recv
+  for range COLL { }   → (list :range-for nil nil nil COLL BODY)
+  for k, v := range C  → (list :range-for :short-decl KEY VAL COLL BODY)
+  ```
+
+  **Design insight for the kit**: the `:go` and `:defer` shapes are
+  pleasingly minimal — both wrap a single expression. Erlang's
+  `spawn(Mod, Fun, Args)` will produce something more elaborate; the
+  scheduler kit primitive `(sched-spawn task)` should accept a thunk so
+  both languages reduce to a uniform spawn API.
+
+  The `:send` shape carries CHAN + VALUE — symmetric with channel-recv
+  as the unary `<-` form. Once the scheduler has channel primitives,
+  both shapes thunk-down to a single `(chan-op direction chan value)`
+  abstraction.
+
+  Range over channels (`for v := range ch`) is currently parsed as
+  range-for with `coll = ch`; the scheduler kit will dispatch on the
+  type of `coll` at execution time (channels yield via receive,
+  collections via iteration). This dispatch is the right place for the
+  scheduler kit to express the channel-receive ⇄ iteration polymorphism.
+  Source: Go-on-SX commit `parse.sx — go/defer/send/range`.
+
 - 2026-05-26 — Plan drafted. Phase 0 unstarted. Awaiting Go-on-SX to begin
   Phase 1.