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/tests/runtime.sx

@@ -103,6 +103,55 @@
   20)
 
 ;; ── report ─────────────────────────────────────────────────────
+(go-rt-test
+  "select: default runs when no case is ready"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "x := 0") (go-parse "select { case <-ch: x = 1 ; default: x = 99 }")))))
+    (go-env-lookup env "x"))
+  99)
+
+(go-rt-test
+  "select: recv case fires when ready"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "ch <- 7") (go-parse "x := 0") (go-parse "select { case <-ch: x = 1 ; default: x = 99 }")))))
+    (go-env-lookup env "x"))
+  1)
+
+(go-rt-test
+  "select: recv-into-var binds the value"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "ch <- 42") (go-parse "select { case v := <-ch: v }")))))
+    (go-env-lookup env "v"))
+  42)
+
+(go-rt-test
+  "select: send case (always ready in v0)"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "select { case ch <- 5: }")))))
+    (go-chan-len (go-env-lookup env "ch")))
+  1)
+
+(go-rt-test
+  "select: picks first ready case"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "a := make()") (go-parse "b := make()") (go-parse "b <- 100") (go-parse "x := 0") (go-parse "select { case <-a: x = 1 ; case <-b: x = 2 ; default: x = 99 }")))))
+    (go-env-lookup env "x"))
+  2)
+
+(go-rt-test
+  "select: no default + nothing ready → blocked error"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()")))))
+    (go-eval-stmt env (go-parse "select { case <-ch: }") (list)))
+  (list :eval-error :select-blocked-no-default))
+
+(go-rt-test
+  "select: combined with goroutine fan-in"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "func push(c chan int, v int) { c <- v }") (go-parse "ch := make()") (go-parse "go push(ch, 7)") (go-parse "result := 0") (go-parse "select { case v := <-ch: result = v ; default: result = -1 }")))))
+    (go-env-lookup env "result"))
+  7)
+
 (define
   go-rt-test-summary
   (str "runtime " go-rt-test-pass "/" go-rt-test-count))