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.md

@@ -1,6 +1,6 @@
 # Go-on-SX Scoreboard
 
-**Total: 556 / 556 tests passing**
+**Total: 597 / 597 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -9,7 +9,7 @@
 | ✅ | types | 102 | 102 |
 | ✅ | eval | 106 | 106 |
 | ✅ | runtime | 40 | 40 |
-| ⬜ | stdlib | 0 | 0 |
+| ✅ | stdlib | 41 | 41 |
 | ⬜ | e2e | 0 | 0 |
 
 Generated by `lib/go/conformance.sh`.