go: eval.sx — maps + index-assign + 8 tests; word-count e2e [nothing]

Phase 4 cont. Adds map values and index-assignment for both
slices and maps.

Map representation: (list :go-map ENTRIES) where ENTRIES is an
association list of (key value) pairs.

  go-map-get / go-map-set    — primitive lookup + functional-update.
  go-slice-set               — same idea for slices.

go-extract-map-entries reads each :kv element in a composite literal,
evaluating key and value. go-eval-composite dispatches on :ty-map to
build the :go-map value.

go-eval-index extended: when OBJ is a :go-map, look up the key via
go-map-get. Missing keys return nil in v0 (Go's real semantics is
the zero value of the value type — needs runtime type info that this
slice doesn't yet thread through).

go-eval-builtin's len handles :go-map alongside :go-slice and strings.

go-eval-assign-pairs gets a new branch for (:index OBJ IDX) LHS:
  - var-rooted indexing only (a[i] = v / m["k"] = v)
  - slice → go-slice-set then rebind the var
  - map   → go-map-set then rebind the var

**Word-counter via map[string]int works end-to-end:**

  words := []string{"a", "b", "a", "c", "a"}
  counts := map[string]int{}
  for i := 0; i < len(words); i++ {
    counts[words[i]] = counts[words[i]] + 1
  }
  // counts["a"] == 3

Builds on:
  - map composite literal eval
  - map index lookup
  - map index-assign
  - slice indexing
  - len() builtin
  - nil + 1 = 1 (numeric-coercion of missing-key default)

eval 58/58, total 435/435.

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

lib/go/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "language": "go",
-  "total_pass": 427,
-  "total": 427,
+  "total_pass": 435,
+  "total": 435,
   "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":50,"total":50,"status":"ok"},
+    {"name":"eval","pass":58,"total":58,"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"}