go: eval.sx + sched.sx — select stmt evaluation + 6 tests [nothing]

Phase 5 cont. Adds `select` statement evaluation:

  go-select-try-case env COMM →
    :not-ready / extended-env / :eval-error
  go-select-pick env CASES DEFAULT-OR-NIL →
    body-result / blocked-error
  go-eval-select-stmt env STMT  — public entry

Walks cases in declared order:
  * :send case — always ready in v0 (unbounded buffer). Sends value
    via go-chan-send! and returns env unchanged.
  * :short-decl / :assign case — RHS expected to be unary <- on a
    channel. Ready iff go-chan-len > 0; on success, recv-into-var
    binds the new value in env.
  * Bare recv (:app (:var "<-") [CHAN]) — ready iff len > 0; consumes
    the value (discarded).
  * :default — deferred until end of walk. Runs if no other case
    ready. Absence + no ready case → (:eval-error :select-blocked-
    no-default).

New `go-chan-len` accessor on the channel closure-bundle so the
select can peek without consuming.

Subtle bug fix: the :select stmt branch in go-eval-stmt was returning
the old env instead of the env returned by the case body. Assignments
inside select cases (`select { case <-ch: x = 1 ; default: x = 99 }`)
now stick.

Tests (6):
  default fires when no case ready
  recv case fires when ready
  recv-into-var binds the value
  send case always ready
  picks first ready case (deterministic order in v0)
  no default + nothing ready → blocked error
  combined with goroutine fan-in

runtime 18/18, total 475/475.

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

lib/go/sched.sx

@@ -50,7 +50,8 @@
             :empty :else
             (let ((v (first buf))) (set! buf (rest buf)) v)))
         (fn () closed)
-        (fn () (set! closed true) nil)))))
+        (fn () (set! closed true) nil)
+        (fn () (len buf))))))
 
 (define
   go-chan?
@@ -62,3 +63,4 @@
 (define go-chan-recv! (fn (ch) ((nth ch 2))))
 (define go-chan-closed? (fn (ch) ((nth ch 3))))
 (define go-chan-close! (fn (ch) ((nth ch 4))))
+(define go-chan-len    (fn (ch) ((nth ch 5))))