feat(p0.5): continuous-single navigation math (current-page + scroll clamp)

Extends PageStackMetrics with the navigation geometry continuous-single needs:
maxScrollExtent (last page bottom rests at viewport bottom, never negative),
clampScroll, and dominantPageAt — the page covering most of the viewport, which
drives the page-number indicator + thumbnail-grid highlight + jump-to-page (F4).
Pure; clamps past both ends; 0 for empty documents.

flutter analyze lib/editor clean; 129/129 tests (+6).

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

lib/editor/layout/page_viewport.dart

@@ -114,6 +114,49 @@ class PageStackMetrics {
     return PageWindow(first, last);
   }
 
+  /// Maximum scroll offset so the last page bottom rests at the viewport
+  /// bottom (never negative — a document shorter than the viewport can't
+  /// scroll).
+  double maxScrollExtent(double viewportExtent) {
+    final max = totalExtent - viewportExtent;
+    return max > 0 ? max : 0.0;
+  }
+
+  /// Clamps [scrollOffset] into the legal `[0, maxScrollExtent]` range.
+  double clampScroll(double scrollOffset, double viewportExtent) {
+    final max = maxScrollExtent(viewportExtent);
+    if (scrollOffset < 0) return 0.0;
+    return scrollOffset > max ? max : scrollOffset;
+  }
+
+  /// The "current" page for a scroll position: the page covering the LARGEST
+  /// portion of the viewport `[scrollOffset, scrollOffset + viewportExtent)`.
+  /// Drives the page-number indicator + thumbnail-grid highlight (F4). Returns
+  /// 0 for an empty document.
+  int dominantPageAt(double scrollOffset, double viewportExtent) {
+    if (_heights.isEmpty) return 0;
+    final window = visibleRange(scrollOffset, viewportExtent);
+    if (window.isEmpty) {
+      // Past the end / before the start → clamp to nearest real page.
+      return scrollOffset <= 0 ? 0 : pageCount - 1;
+    }
+    final viewTop = scrollOffset;
+    final viewBottom = scrollOffset + viewportExtent;
+    var best = window.first;
+    var bestOverlap = -1.0;
+    for (var i = window.first; i <= window.last; i++) {
+      final top = _tops[i];
+      final bottom = top + _heights[i];
+      final overlap =
+          math.min(bottom, viewBottom) - math.max(top, viewTop);
+      if (overlap > bestOverlap) {
+        bestOverlap = overlap;
+        best = i;
+      }
+    }
+    return best;
+  }
+
   /// Lowest index whose band bottom is strictly after [y] (i.e. the first page
   /// that the window's top edge does not sit fully below).
   int _firstIntersecting(double y) {