go: eval.sx — for / break / continue / inc-dec + 7 tests [nothing]

Phase 4 cont. go-eval-for handles all three for-header shapes:

  for { ... }                          — infinite (cond defaults to true)
  for cond { ... }                     — while-like (init=nil, post=nil)
  for init ; cond ; post { ... }       — C-style

Implementation:
  * Run INIT (if any), extending env.
  * Loop: eval COND. If false, exit with current env.
    Eval body (a :block). Catch sentinels:
      :return-value → propagate up
      :break        → exit loop with pre-break env
      :continue     → still runs POST, then re-loops
    Otherwise: run POST, re-loop.

:break and :continue propagate as keyword sentinels through
go-eval-block alongside the existing :return-value sentinel. The
block returns whichever sentinel hit first; control-flow constructs
(for, switch, select) catch them.

inc-dec (x++ / x--) updates env via the same shadowing model used by
assign — `(go-env-extend env name (+ current 1))`.

**Iterative fact(5) = 120 and the classic sum-to-9 = 45 both
evaluate.** Demonstrates the for-loop machinery is solid enough for
real programs.

eval 40/40, total 417/417.

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: 410 / 410 tests passing**
+**Total: 417 / 417 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
 | ✅ | parse | 176 | 176 |
 | ✅ | types | 72 | 72 |
-| ✅ | eval | 33 | 33 |
+| ✅ | eval | 40 | 40 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |