go: eval.sx — range-over-{slice,map,chan} + 7 tests; break-env fix [nothing]

Phase 5 cont. New go-eval-range-for handles the parser's :range-for
AST shape. Dispatches on the collection's runtime type:

  :go-slice  → bind index + element, iterate by position
  :go-map    → bind key + value, walk entries assoc list
  :go-chan   → bind value, drain until buffer empty (v0 limitation)

Each loop carries:
  - go-range-extend: handles 0/1/2-name binding patterns uniformly
  - go-range-body:   evaluates body whether it's a :block or other shape
  - per-collection loop helper: threads env, catches :break/:continue/
    :return-value/:eval-error sentinels

**Subtle break fix:** loops were previously returning the *pre-loop*
env when break fired, clobbering all assignments made in prior
iterations. Now returns the current iteration's input env (which
carries forward successful iterations' state). Patched for the three
range variants and for the regular for-loop where the same pattern
applied. The shape:

  (= r :break) env    ;; was: (= r :break) original-env

Tests:
  range: slice — sum of 1..5 = 15
  range: slice — key only (index)
  range: map — sum values
  range: channel — collect all buffered
  range: slice with break exits early
  range: slice with continue skips an element
  range: empty slice — body never runs
  range: chan + goroutine producer

runtime 26/26, total 483/483.

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: 475 / 475 tests passing**
+**Total: 483 / 483 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -8,7 +8,7 @@
 | ✅ | parse | 176 | 176 |
 | ✅ | types | 72 | 72 |
 | ✅ | eval | 80 | 80 |
-| ✅ | runtime | 18 | 18 |
+| ✅ | runtime | 26 | 26 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |