docs(control-flow): explain seenStack push rationale (matches upstream ember-template-lint design)

Author: johanrd

lib/utils/control-flow.js

@@ -185,6 +185,14 @@ function createConditionalScope() {
           conditionalStack.at(-1).add(key);
         }
       } else {
+        // Top-level conditional ended — its accumulated keys are now
+        // unconditionally "seen" at this scope. Push as a separate frame
+        // (rather than merging into seenStack[0]) to mirror upstream
+        // ember-template-lint's ConditionalScope (PR #1606) and keep
+        // per-conditional grouping for future debugging hooks. Functionally
+        // equivalent to a single merged Set for `has()` purposes; growth is
+        // O(top-level conditionals per template), which is bounded by source
+        // size.
         seenStack.push(keys);
       }
     },