go: eval.sx — slices + index + slice expr + len/append builtins + 10 tests [nothing]

Phase 4 cont. Adds runtime support for Go's slice type.

Slice representation: (list :go-slice ELEMS) — a simple wrapper around
a list of element values. v0 deferring the full
(length, capacity, backing-vector) triple from the Go spec until
programs need it.

  go-eval-composite      → for (:composite TYPE-OR-EXPR ELEMS) where
                            TYPE is :ty-slice / :ty-array, eval each
                            element (handling :kv index-keyed
                            shorthand by taking only the value) and
                            wrap in :go-slice.
  go-eval-index          → (:index OBJ IDX). Bounds-checked; out-of-
                            range returns (:eval-error :index-out-of-range).
  go-eval-slice          → (:slice OBJ LOW HIGH MAX). Two-index slice
                            with omitted low → 0, omitted high → len.
                            Returns a new :go-slice.
  go-list-slice          → primitive list-slicing helper.

Builtins live in a new starter env go-env-builtins:
  len(slice|string)      → count
  append(slice, ...x)    → new slice with x appended
  print(...)             → no-op in v0

Builtins are bound as (:go-builtin NAME); go-eval-call recognises the
shape and routes to go-eval-builtin instead of go-eval-fn.

**Summing a slice via the canonical Go for-loop works end-to-end:**

  a := []int{1, 2, 3, 4, 5}
  sum := 0
  for i := 0; i < len(a); i++ {
    sum = sum + a[i]
  }
  // sum == 15

eval 50/50, total 427/427.

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

lib/go/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "language": "go",
-  "total_pass": 417,
-  "total": 417,
+  "total_pass": 427,
+  "total": 427,
   "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":40,"total":40,"status":"ok"},
+    {"name":"eval","pass":50,"total":50,"status":"ok"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}