Add change-header-lines action to dynamically change --header-lines

All input lines now enter the chunklist with sequential indices, and
header lines are excluded from matching via Pattern.startIndex and
PassMerger offset. This allows the number of header lines to be changed
at runtime with change-header-lines(N), transform-header-lines, and
bg-transform-header-lines actions.

- Remove EvtHeader event; header items are read directly from chunks
- Add startIndex to Pattern and PassMerger for skipping header items
- Add targetIndex field to Terminal for cursor repositioning across
  header-lines changes

Close #4659

test/test_core.rb

@@ -2176,6 +2176,80 @@ class TestCore < TestInteractive
     end
   end
 
+  def test_change_header_lines
+    tmux.send_keys %(seq 10 | #{FZF} --header-lines 3 --bind 'space:change-header-lines(5),enter:transform-header-lines(echo 1)'), :Enter
+    tmux.until do |lines|
+      assert_equal 7, lines.item_count
+      assert lines.any_include?('> 4')
+    end
+    tmux.send_keys :Space
+    tmux.until do |lines|
+      assert_equal 5, lines.item_count
+      assert lines.any_include?('> 6')
+    end
+    tmux.send_keys :Enter
+    tmux.until do |lines|
+      assert_equal 9, lines.item_count
+      assert lines.any_include?('> 6')
+    end
+  end
+
+  def test_change_header_lines_to_zero
+    tmux.send_keys %(seq 5 | #{FZF} --header-lines 3 --bind 'space:bg-transform-header-lines(echo 0)'), :Enter
+    tmux.until do |lines|
+      assert_equal 2, lines.item_count
+      assert lines.any_include?('> 4')
+    end
+    tmux.send_keys :Space
+    tmux.until do |lines|
+      assert_equal 5, lines.item_count
+      # All items are now in the list, cursor stays on item 4
+      assert lines.any_include?('> 4')
+    end
+  end
+
+  def test_change_header_lines_deselect
+    # Selected items that become part of the header should be deselected
+    tmux.send_keys %(seq 10 | #{FZF} --multi --header-lines 0 --bind 'space:change-header-lines(3),enter:change-header-lines(1)'), :Enter
+    tmux.until do |lines|
+      assert_equal 10, lines.item_count
+      assert lines.any_include?('> 1')
+    end
+    # Select items 1, 2, 3 (these will become header lines)
+    tmux.send_keys :BTab, :BTab, :BTab
+    tmux.until { |lines| assert_equal 3, lines.select_count }
+    # Also select item 4 (this should remain selected)
+    tmux.send_keys :BTab
+    tmux.until { |lines| assert_equal 4, lines.select_count }
+    # Change header-lines to 3: items 1, 2, 3 become headers and should be deselected
+    tmux.send_keys :Space
+    tmux.until do |lines|
+      assert_equal 7, lines.item_count
+      assert_equal 1, lines.select_count
+      assert lines.any_include?('> 5')
+    end
+    # Change header-lines to 1
+    tmux.send_keys :Enter
+    tmux.until do |lines|
+      assert_equal 9, lines.item_count
+      assert_equal 1, lines.select_count
+      assert lines.any_include?('> 5')
+    end
+  end
+
+  def test_change_header_lines_reverse
+    tmux.send_keys %(seq 10 | #{FZF} --header-lines 2 --reverse --bind 'space:change-header-lines(4)'), :Enter
+    tmux.until do |lines|
+      assert_equal 8, lines.item_count
+      assert lines.any_include?('> 3')
+    end
+    tmux.send_keys :Space
+    tmux.until do |lines|
+      assert_equal 6, lines.item_count
+      assert lines.any_include?('> 5')
+    end
+  end
+
   def test_zero_width_characters
     tmux.send_keys %(for i in {1..1000}; do string+="a̱$i"; printf '\\e[43m%s\\e[0m\\n' "$string"; done | #{FZF} --ansi --query a500 --ellipsis XX), :Enter
     tmux.until do |lines|