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/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "language": "go",
-  "total_pass": 469,
-  "total": 469,
+  "total_pass": 475,
+  "total": 475,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"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":12,"total":12,"status":"ok"},
+    {"name":"runtime","pass":18,"total":18,"status":"ok"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
   ]