docs(render): Phase U.2a review fixups — LIFO-on-ties comment + ISSUES #102

Code-review minor follow-ups: correct the CellTodoList comments (ties are LIFO,
not FIFO — an equal-distance newcomer lands at the tail and pops first, matching
retail's break-on-first-not-greater + pop-from-tail). Update ISSUES #102 to record
that U.2a closes I-1/I-2 (under-count + duplicate accumulation) via the enqueue-once
gate, narrowing the residual to diamond-topology clip-completeness (AddToCell onward
re-propagation, tracked under U.6).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/PortalVisibilityBuilder.cs

@@ -268,7 +268,8 @@ public static class PortalVisibilityBuilder
     /// the tail; <see cref="PopNearest"/> removes the tail — giving closest-first traversal exactly
     /// as ConstructView's pop-from-(cell_todo_num-1) does (433767-433769). The insertion only shifts
     /// entries strictly farther than the newcomer (retail's flag test breaks on the first
-    /// not-greater entry), so ties preserve insertion order (stable, FIFO among equal distances).
+    /// not-greater entry), so an equal-distance newcomer lands at the tail and pops FIRST —
+    /// LIFO on ties, matching retail's break-on-first-not-greater + pop-from-tail.
     /// </summary>
     private sealed class CellTodoList
     {
@@ -280,7 +281,8 @@ public static class PortalVisibilityBuilder
         {
             // Find the slot: scan from the tail (nearest) toward the head while existing entries are
             // strictly nearer than `distance`, so the newcomer lands just ABOVE every entry that is
-            // farther-or-equal — i.e. nearest-at-tail order, FIFO on ties.
+            // farther-or-equal — i.e. nearest-at-tail order, LIFO on ties (an equal-distance
+            // newcomer inserts at the tail and pops first).
             int idx = _items.Count;
             while (idx > 0 && _items[idx - 1].Distance < distance)
                 idx--;