fix: scroll mark-as-read — delta filter and threshold tuned from logs

EVIDENCE (from xcode.txt):
- down=false ALWAYS: per-entry deltas are ~1-2pt per callback,
  but filter required >2pt. Every delta was rejected.
- cumDown stuck at 129: threshold was max(100, 956*0.2) = 191.
  With most deltas rejected, cumulative barely grew.

FIXES:
1. Delta filter: >2pt → >0.5pt for direction detection.
   Cumulative accumulation accepts any delta >0 (no filter).
   Per-entry callbacks deliver small deltas — filtering at 2pt
   discarded virtually all genuine scroll events.

2. Threshold: removed 20% viewport scaling, fixed at 100pt.
   The scaling made sense for a global offset tracker (large
   deltas), not per-entry tracking (small deltas).

Removed debug logging.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/Features/Reader/Views/EntryListView.swift

@@ -15,8 +15,9 @@ struct EntryListView: View {
     // Safe because this view only appears on iPhone in portrait/landscape.
     private var viewportHeight: CGFloat { UIScreen.main.bounds.height }
 
-    // Dynamic threshold: max(100pt, 20% of viewport)
-    private var activationThreshold: CGFloat { max(100, viewportHeight * 0.2) }
+    // Fixed threshold — per-entry deltas are small (~1-2pt each),
+    // so 100pt accumulates after scrolling roughly one screenful.
+    private let activationThreshold: CGFloat = 100
 
     var body: some View {
         if vm.isLoading && vm.entries.isEmpty {
@@ -73,8 +74,10 @@ struct EntryListView: View {
                             let isScrollingDown: Bool
                             if let prev = prevMinY {
                                 let delta = prev - newMinY // positive = scrolling down
-                                isScrollingDown = delta > 2
-                                if isScrollingDown {
+                                // Per-entry deltas are small (~1-2pt per callback).
+                                // Accept anything > 0.5 as genuine downward scroll.
+                                isScrollingDown = delta > 0.5
+                                if delta > 0 {
                                     cumulativeDown += delta
                                     if cumulativeDown > activationThreshold {
                                         trackingActive = true
@@ -96,17 +99,12 @@ struct EntryListView: View {
                             }
 
                             // --- Mark-as-read ---
-                            let debugThis = vm.entries.prefix(3).contains(where: { $0.id == entryId })
-                            if debugThis {
-                                print("[SCROLL] id=\(entryId) minY=\(Int(newMinY)) maxY=\(Int(frame.maxY)) down=\(isScrollingDown) active=\(trackingActive) cumDown=\(Int(cumulativeDown)) wasVis=\(wasVisible.contains(entryId)) visR=\(String(format:"%.2f",visibleRatio)) vpH=\(Int(viewportHeight)) read=\(entry.isRead) marked=\(markedByScroll.contains(entryId))")
-                            }
                             guard trackingActive,
                                   isScrollingDown,
                                   !entry.isRead,
                                   !markedByScroll.contains(entryId),
                                   wasVisible.contains(entryId),
                                   frame.maxY < 0 else { return }
-                            if debugThis { print("[SCROLL-READ] ✅ id=\(entryId)") }
 
                             markedByScroll.insert(entryId)