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/scoreboard.json

@@ -1,14 +1,14 @@
 {
   "language": "go",
-  "total_pass": 556,
-  "total": 556,
+  "total_pass": 597,
+  "total": 597,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":179,"total":179,"status":"ok"},
     {"name":"types","pass":102,"total":102,"status":"ok"},
     {"name":"eval","pass":106,"total":106,"status":"ok"},
     {"name":"runtime","pass":40,"total":40,"status":"ok"},
-    {"name":"stdlib","pass":0,"total":0,"status":"pending"},
+    {"name":"stdlib","pass":41,"total":41,"status":"ok"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
   ]
 }