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/conformance.sh

@@ -32,6 +32,7 @@ SUITES=(
   "types|go-types-test-pass|go-types-test-count"
   "eval|go-eval-test-pass|go-eval-test-count"
   "runtime|go-rt-test-pass|go-rt-test-count"
+  "stdlib|go-std-test-pass|go-std-test-count"
 )
 
 cat > "$TMPFILE" <<'EPOCHS'
@@ -44,11 +45,14 @@ cat > "$TMPFILE" <<'EPOCHS'
 (load "lib/go/types.sx")
 (load "lib/go/sched.sx")
 (load "lib/go/eval.sx")
+(load "lib/go/std/strings.sx")
+(load "lib/go/std/strconv.sx")
 (load "lib/go/tests/lex.sx")
 (load "lib/go/tests/parse.sx")
 (load "lib/go/tests/types.sx")
 (load "lib/go/tests/eval.sx")
 (load "lib/go/tests/runtime.sx")
+(load "lib/go/tests/stdlib.sx")
 EPOCHS
 
 idx=0
@@ -113,7 +117,6 @@ cat > lib/go/scoreboard.json <<JSON
   "total_pass": $TOTAL_PASS,
   "total": $TOTAL_COUNT,
   "suites": [$JSON_SUITES,
-    {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
   ]
 }
@@ -126,8 +129,7 @@ cat > lib/go/scoreboard.md <<MD
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
-$MD_ROWS| ⬜ | stdlib | 0 | 0 |
-| ⬜ | e2e | 0 | 0 |
+$MD_ROWS| ⬜ | e2e | 0 | 0 |
 
 Generated by \`lib/go/conformance.sh\`.
 MD