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>

lib/go/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "language": "go",
-  "total_pass": 281,
-  "total": 281,
+  "total_pass": 290,
+  "total": 290,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
-    {"name":"parse","pass":152,"total":152,"status":"ok"},
+    {"name":"parse","pass":161,"total":161,"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"},