Add LAZYGIT_SLOW_RENDER debug knob for watching async render frames

Re-rendering a diff into a main view is asynchronous and lazy: the read
loop fills the view a screenful at a time and refreshes as it goes. When
debugging scroll-restore and flicker behaviour, the individual frames go by
too fast to see. Setting LAZYGIT_SLOW_RENDER=<milliseconds> sleeps that long
after each line is written, stretching the load out so the frames become
visible. It has no effect when unset, so it's safe to leave in.

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

pkg/tasks/tasks.go

@@ -4,7 +4,9 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"os"
 	"os/exec"
+	"strconv"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -313,6 +315,17 @@ func (self *ViewBufferManager) NewCmdTask(start func() (Cmd, io.Reader), prefix
 			// this to work out how many more lines, if any, we still need to read.
 			linesRead := 0
 
+			// Set LAZYGIT_SLOW_RENDER=<milliseconds> to sleep that long after each
+			// line is written to the view, stretching async loads out so the frames
+			// of a re-render become visible. Useful for debugging scroll/flicker
+			// behaviour; has no effect when the variable is unset.
+			var slowRenderPerLine time.Duration
+			if v := os.Getenv("LAZYGIT_SLOW_RENDER"); v != "" {
+				if ms, err := strconv.Atoi(v); err == nil {
+					slowRenderPerLine = time.Duration(ms) * time.Millisecond
+				}
+			}
+
 		outer:
 			for {
 				if stopped() {
@@ -369,6 +382,10 @@ func (self *ViewBufferManager) NewCmdTask(start func() (Cmd, io.Reader), prefix
 						lineWrittenChan <- struct{}{}
 						linesRead++
 
+						if slowRenderPerLine > 0 {
+							time.Sleep(slowRenderPerLine)
+						}
+
 						if linesRead == linesToRead.InitialRefreshAfter {
 							// We have read enough lines to fill the view, so do a first refresh
 							// here to show what we have. Continue reading and refresh again at