go: eval.sx scaffold — literals + vars + binops + 25 tests; Phase 3 closed [nothing]

Phase 3 — bidirectional type checker — is fully ticked (short-decl
was already implemented). Phase 4 starts here.

lib/go/eval.sx single judgment:

  (go-eval ENV EXPR)  →  VALUE | (list :eval-error TAG ...)

ENV is an association list of (NAME VALUE) bindings — same shape as
the type checker's ctx, but the entries are runtime values. Values
are represented directly in SX: integers/floats as SX numbers,
strings as SX strings, booleans as true/false, nil as nil. Composite
values (slices/maps/structs/pointers/channels) arrive in later slices.

First-slice coverage:

  * go-env-empty / -lookup / -extend
  * Literal decoding:
      decimal (with underscores)
      hex (0x.. / 0X..)
      oct (0o.. / 0O..)
      bin (0b.. / 0B..)
    via go-hex-digit-value (explicit char equality — SX's nth on
    strings returns single-char strings, not numeric codes; the
    arithmetic-on-char-codes pattern from the OCaml kernel ports
    doesn't work here).
  * Identifier lookup with predeclared true / false / nil.
  * Binops: + - * / and the six comparison ops and && / ||.
  * Errors as (:eval-error TAG ...) sentinels.

Statements (block / return / short-decl / assign), control flow
(if / for), and function application / closures arrive in subsequent
slices.

eval 25/25, total 402/402.

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

lib/go/conformance.sh

@@ -30,6 +30,7 @@ SUITES=(
   "lex|go-test-pass|go-test-count"
   "parse|go-parse-test-pass|go-parse-test-count"
   "types|go-types-test-pass|go-types-test-count"
+  "eval|go-eval-test-pass|go-eval-test-count"
 )
 
 cat > "$TMPFILE" <<'EPOCHS'
@@ -40,9 +41,11 @@ cat > "$TMPFILE" <<'EPOCHS'
 (load "lib/go/lex.sx")
 (load "lib/go/parse.sx")
 (load "lib/go/types.sx")
+(load "lib/go/eval.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")
 EPOCHS
 
 idx=0
@@ -107,7 +110,6 @@ cat > lib/go/scoreboard.json <<JSON
   "total_pass": $TOTAL_PASS,
   "total": $TOTAL_COUNT,
   "suites": [$JSON_SUITES,
-    {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
@@ -122,8 +124,7 @@ cat > lib/go/scoreboard.md <<MD
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
-$MD_ROWS| ⬜ | eval | 0 | 0 |
-| ⬜ | runtime | 0 | 0 |
+$MD_ROWS| ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |