perf: share RowGroup instances across ScenarioData snapshots

RowGroup is effectively immutable — all fourteen fields are declared
final and no caller mutates the lists returned by its getters (getRows()
even wraps its internal list in Collections.unmodifiableList()). The
deep-copy loop in RowGroupSequence that called new RowGroup(rg) for
every element was therefore pure waste: for a scenario-heavy sequence
it could produce thousands of unnecessary RowGroup objects and four
times as many ArrayList allocations per layout pass.

Replace the per-element copy loop with new ArrayList<>(template.group),
which copies the list container (so independent add() calls on one
snapshot do not affect another) while sharing the RowGroup element
objects. Add a null guard to match the behaviour of the non-deepMode
path.

Remove the now-unreachable RowGroup(RowGroup template) copy constructor
to prevent future confusion.

Note: the blocks list is intentionally still shallow-copied in deepMode.
ScenarioData.getBlocks().add() mutates it after snapshots are taken, so
sharing it would corrupt saved states (confirmed by three integration
test failures during development).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: kalaspuffar

src/org/daisy/dotify/formatter/impl/page/RowGroup.java

@@ -171,30 +171,6 @@ private RowGroup(Builder builder) {
         this.lineProps = builder.lineProps;
     }
 
-    /**
-     * Creates a deep copy of the supplied instance.
-     *
-     * @param template the instance to copy
-     */
-    RowGroup(RowGroup template) {
-        this.rows = new ArrayList<>(template.rows);
-        this.markers = new ArrayList<>(template.markers);
-        this.identifiers = new ArrayList<>(template.identifiers);
-        this.breakable = template.breakable;
-        this.skippable = template.skippable;
-        this.collapsible = template.collapsible;
-        this.unitSize = template.unitSize;
-        this.lastUnitSize = template.lastUnitSize;
-        this.ids = new ArrayList<>(template.ids);
-        this.lazyCollapse = template.lazyCollapse;
-        this.keepWithNextSheets = template.keepWithNextSheets;
-        this.keepWithPreviousSheets = template.keepWithPreviousSheets;
-        this.avoidVolumeBreakAfterPriority = template.avoidVolumeBreakAfterPriority;
-        this.lastInBlock = template.lastInBlock;
-        this.mergeable = template.mergeable;
-        this.lineProps = template.lineProps;
-    }
-
     private static float getRowSpacing(float rowDefault, RowImpl r) {
         return (r.getRowSpacing() != null ? r.getRowSpacing() : rowDefault);
     }

src/org/daisy/dotify/formatter/impl/page/RowGroupSequence.java

@@ -49,10 +49,10 @@ public RowGroupSequence(
     private RowGroupSequence(RowGroupSequence template, VerticalSpacing vs, int offset, boolean deepMode) {
         this.blocks = deepMode ? new ArrayList<>(template.blocks) : template.blocks;
         if (deepMode) {
-            this.group = new ArrayList<>();
-            for (RowGroup rg : template.group) {
-                group.add(new RowGroup(rg));
-            }
+            // RowGroup is effectively immutable (all fields are final and no caller mutates the
+            // lists returned by its getters), so the element objects can be shared.  A new list
+            // container is required so that add() calls on one snapshot do not affect another.
+            this.group = template.group == null ? null : new ArrayList<>(template.group);
         } else {
             if (template.group == null) {
                 this.group = null;