go: eval.sx — stmts + function application; recursive fib evaluates + 8 tests [nothing]

Phase 4 cont. go-eval-stmt dispatches on:
  :return       → wraps value in (:return-value V) sentinel
  :var-decl     → bind each NAME via go-eval-var-decl
  :short-decl   → bind each (:var NAME) lhs to corresponding expr value
  :assign       → immutable-env shadowing (true mutation deferred)
  :block        → run stmts via go-eval-block, propagating :return-value
  :if / :else   → cond-driven dispatch
  :func-decl    → bind name to (list :go-fn PARAMS BODY)
  else          → expression statement, evaluate for side effects

go-eval-call extends the CALLER's env with param-names → arg-values
(dynamic-scope-ish — closures don't capture lexical env yet), runs the
body block, catches :return-value and unwraps.

**Recursive fib(5) = 5 evaluates correctly.** Recursion works because
top-level func bindings are in the calling env before the recursive
call happens.

True lexical closures (let bind sees outer var; assignments visible to
nested funcs) need an env-cell model with mutation; deferred to a
later slice.

eval 33/33, total 410/410.

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

lib/go/scoreboard.md

@@ -1,13 +1,13 @@
 # Go-on-SX Scoreboard
 
-**Total: 402 / 402 tests passing**
+**Total: 410 / 410 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
 | ✅ | parse | 176 | 176 |
 | ✅ | types | 72 | 72 |
-| ✅ | eval | 25 | 25 |
+| ✅ | eval | 33 | 33 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |