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/conformance.sh

@@ -31,6 +31,7 @@ SUITES=(
   "parse|go-parse-test-pass|go-parse-test-count"
   "types|go-types-test-pass|go-types-test-count"
   "eval|go-eval-test-pass|go-eval-test-count"
+  "runtime|go-rt-test-pass|go-rt-test-count"
 )
 
 cat > "$TMPFILE" <<'EPOCHS'
@@ -41,11 +42,13 @@ cat > "$TMPFILE" <<'EPOCHS'
 (load "lib/go/lex.sx")
 (load "lib/go/parse.sx")
 (load "lib/go/types.sx")
+(load "lib/go/sched.sx")
 (load "lib/go/eval.sx")
 (load "lib/go/tests/lex.sx")
 (load "lib/go/tests/parse.sx")
 (load "lib/go/tests/types.sx")
 (load "lib/go/tests/eval.sx")
+(load "lib/go/tests/runtime.sx")
 EPOCHS
 
 idx=0
@@ -110,7 +113,6 @@ cat > lib/go/scoreboard.json <<JSON
   "total_pass": $TOTAL_PASS,
   "total": $TOTAL_COUNT,
   "suites": [$JSON_SUITES,
-    {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
   ]
@@ -124,8 +126,7 @@ cat > lib/go/scoreboard.md <<MD
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
-$MD_ROWS| ⬜ | runtime | 0 | 0 |
-| ⬜ | stdlib | 0 | 0 |
+$MD_ROWS| ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |
 
 Generated by \`lib/go/conformance.sh\`.