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>

docs/ISSUES.md

@@ -84,11 +84,32 @@ Related: #102 (builder dungeon-scaling fixpoint).
 
 ## #102 — A8.F PortalVisibilityBuilder — port retail update_count fixpoint (replace MaxReprocessPerCell cap)
 
-**Status:** OPEN
+**Status:** PARTIALLY RESOLVED (Phase U.2a, 2026-05-30, commit `d880775`)
-**Severity:** MEDIUM
+**Severity:** MEDIUM → LOW (residual is diamond-topology clip-completeness only)
 **Filed:** 2026-05-29
 **Component:** rendering, visibility, EnvCell portal traversal
 
+**U.2a resolution (2026-05-30):** Reading the decomp showed retail does NOT
+re-enqueue on view-growth: `AddViewToPortals` (433446) enqueues a cell via
+`InsCellTodoList` ONLY in the first-discovery branch (`ecx_5 == 0`); later
+growth goes through `AddToCell` (433050) in place and never re-enqueues. U.2a
+replaced the `MaxReprocessPerCell` cap with an **enqueue-once gate** (a `seen`
+set = retail `cell_view_done`, 433784) + a distance-priority work list (retail
+`InsCellTodoList`). This **closes I-1 and I-2**: the clip-region union into a
+neighbour now runs UNCONDITIONALLY before the enqueue gate, so >4-portal cells
+no longer under-count (I-1 gone), and each cell processes its exit portals
+exactly once, so cyclic graphs no longer accumulate duplicate polygons (I-2
+gone). The new `Build_CyclicHub_TerminatesAndBounds` test enforces the
+acceptance (4-room ring ⇒ ≤5 cells, no dups). **Residual scope:** retail's
+`AddToCell` ONWARD re-propagation of late growth (a cell reached via a longer
+path AFTER it was drawn gets its own `CellView` unioned but does not
+re-propagate that growth to ITS children) is NOT ported — this affects only
+clip-region completeness on **diamond** topologies, never the visible cell set
+or draw order. It overlaps the M-4 `OtherPortalClip` stub below; track both
+under U.6 (dungeon-scale validation). A naive count-watermark re-enqueue is NOT
+a valid fix (it never terminates, because `CellView.Add` appends without
+merging) — the faithful fix is the in-place slice re-propagation.
+
 **Description:** A8.F Task 4 shipped a bounded-BFS port of retail's
 `PView::ConstructView` → `ClipPortals` → `AddViewToPortals` in
 [`src/AcDream.App/Rendering/PortalVisibilityBuilder.cs`](../src/AcDream.App/Rendering/PortalVisibilityBuilder.cs).

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--;