go: sched.sx — channels + goroutines (v0 synchronous) + 12 tests; Phase 5 starts [shapes-scheduler]

Phase 5 (goroutines + channels) opens.

lib/go/sched.sx is the **independent implementation** referenced by
plans/lib-guest-scheduler.md — the first-consumer cut whose realised
shape will inform the eventual sister kit.

Channel representation:
  (list :go-chan SEND-FN RECV-FN CLOSED?-FN CLOSE!-FN)
Each closure shares a mutable `buf` (a list mutated via append! and
set!) and a `closed` flag. Channel identity is closure-instance —
two `make()` calls produce distinct values per Go spec § Channel types.

Primitive API in sched.sx:
  go-make-chan / go-chan? / go-chan-send! / go-chan-recv! /
  go-chan-closed? / go-chan-close!

Eval integration in eval.sx:
  * `make` and `close` added as builtins. v0 `make()` takes no args
    and returns an unbounded-buffer channel.
  * `:send` stmt → go-chan-send! on the channel.
  * Unary `<-` recv on channel values → go-chan-recv!. `:empty`
    sentinel converted to nil (stand-in for blocking semantics).
  * `:go expr` → synchronous eval (v0 limitation, see sched.sx
    header).

**v0 concurrency model — synchronous goroutines.** SX doesn't expose
first-class continuations to guest code, so v0 runs `go f()`
immediately and depends on the spawned goroutine running to
completion before the main goroutine receives. This is the right
semantics for the simple producer/consumer patterns covered here.
True preemption with blocking send/recv is Phase 5b — requires either
a CEK-style trampolining eval rewrite or kit-level continuation
support. Logged in sched.sx header and in the sister-plan diary.

Runtime suite (12 tests):
  * 6 direct API tests: identity, FIFO order, closed-flag
  * 6 source-level: make + send + recv, go ping-pong, close,
    multi-goroutine fan-in, worker-with-result

Sister-plan scheduler diary updated with the channel-as-closure-
bundle insight and the v0 synchronous-spawn caveat.

runtime 12/12, total 469/469.

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

lib/go/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "language": "go",
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-  "total": 457,
+  "total_pass": 469,
+  "total": 469,
   "suites": [
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     {"name":"parse","pass":176,"total":176,"status":"ok"},
     {"name":"types","pass":72,"total":72,"status":"ok"},
     {"name":"eval","pass":80,"total":80,"status":"ok"},
-    {"name":"runtime","pass":0,"total":0,"status":"pending"},
+    {"name":"runtime","pass":12,"total":12,"status":"ok"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
   ]