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/parse.sx

@@ -54,12 +54,17 @@
           (let
             ((ty (gp-tok-type)) (v (gp-tok-value)))
             (cond
+              (or (= ty "string") (= ty "rune"))
+              ;; Quoted/rune literals: tag distinctly so eval/types
+              ;; can dispatch without re-classifying by first-char
+              ;; (which conflates `42` and `"42"`). See lib/go/types.sx
+              ;; `go-classify-literal-string` for the heuristic this
+              ;; tag obviates.
+              (do (gp-advance!) (list :literal-string v))
               (or
                 (= ty "int")
                 (= ty "float")
-                (= ty "imag")
-                (= ty "string")
-                (= ty "rune"))
+                (= ty "imag"))
               (do (gp-advance!) (ast-literal v))
               (= ty "ident")
               (do (gp-advance!) (ast-var v))