results: fix Debug rendering assertion

Check the character in the output string instead of referring to a nonexistent grid_ member. This restores Debug compilation while preserving the intended invariant that rendering must not write an identical character twice.

Size labels intentionally replace interior spaces, so requiring an entirely unwritten destination would reject valid solutions. Update the testing notes now that Debug builds pass.

Tests: Debug and Release CTest suites (4 passed each)

Refs: #7

main.cc

@@ -85,7 +85,8 @@ namespace {
     auto set(std::string &s, size_t x, size_t y, char c) const noexcept -> void {
       assert(x < length_);
       assert(y < length_);
-      assert(grid_[x + y * length_] != c);
+      // Size labels may replace interior spaces, but must not duplicate a write.
+      assert(s[x + y * length_] != c);
       s[x + y * length_] = c;
     }