go: Phase 8 first slice — stdlib strings/strconv, 41 tests, +40 cleared [shapes-static-types-bidirectional]

New :go-package NAME ENTRIES value type with field lookup via
extended go-eval-select. New :go-builtin-fn callable for closure-
based stdlib functions. lib/go/std/strings.sx ships 12 functions
(Contains, HasPrefix, HasSuffix, Index, Count, Repeat, Join,
ToUpper, ToLower, TrimSpace, Split, Replace) + lib/go/std/strconv.sx
ships Itoa/Atoi.

Pre-existing bug fixed: parser was emitting (:literal V) for both
`42` and `"42"`, relying on first-char heuristic in eval/types.
Now emits :literal-string for string/rune literals so Atoi("42")
correctly receives the string. 3 parse tests + 2 in-composite-key
tests updated to new shape.

Total 597/597. Stdlib 41/41 — +40 acceptance bar cleared. Sister
diary documents the 11 value-type kinds (struct/slice/map/chan/
fn/method/builtin/builtin-fn/package/panic/defer) all sharing the
"(:KIND PAYLOAD...)" shape, alongside AST nodes and sentinel signals
as the kit's three orthogonal first-class-tag axes.

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

lib/go/tests/parse.sx

@@ -22,10 +22,10 @@
 (go-parse-test "leading-dot float" (go-parse ".5") (ast-literal ".5"))
 (go-parse-test "exponent float" (go-parse "1e10") (ast-literal "1e10"))
 (go-parse-test "imag literal" (go-parse "2i") (ast-literal "2i"))
-(go-parse-test "string literal" (go-parse "\"hi\"") (ast-literal "hi"))
-(go-parse-test "empty string" (go-parse "\"\"") (ast-literal ""))
-(go-parse-test "raw string" (go-parse "`a\nb`") (ast-literal "a\nb"))
-(go-parse-test "rune literal" (go-parse "'a'") (ast-literal "a"))
+(go-parse-test "string literal" (go-parse "\"hi\"") (list :literal-string "hi"))
+(go-parse-test "empty string" (go-parse "\"\"") (list :literal-string ""))
+(go-parse-test "raw string" (go-parse "`a\nb`") (list :literal-string "a\nb"))
+(go-parse-test "rune literal" (go-parse "'a'") (list :literal-string "a"))
 
 ;; ── primary: identifiers ──────────────────────────────────────────
 (go-parse-test "ident: simple" (go-parse "x") (ast-var "x"))
@@ -262,7 +262,7 @@
 (go-parse-test
   "index: m[\"key\"] (string index)"
   (go-parse "m[\"key\"]")
-  (list :index (ast-var "m") (ast-literal "key")))
+  (list :index (ast-var "m") (list :literal-string "key")))
 
 (go-parse-test
   "index: a[0][1] (chained)"
@@ -691,8 +691,8 @@
   (list
     :composite (list :ty-map (list :ty-name "string") (list :ty-name "int"))
     (list
-      (list :kv (ast-literal "a") (ast-literal "1"))
-      (list :kv (ast-literal "b") (ast-literal "2")))))
+      (list :kv (list :literal-string "a") (ast-literal "1"))
+      (list :kv (list :literal-string "b") (ast-literal "2")))))
 
 (go-parse-test
   "comp: pkg.Point{1, 2} (qualified type)"