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

src/core.go

@@ -17,7 +17,6 @@ Reader   -> EvtReadNew        -> Matcher  (restart)
 Terminal -> EvtSearchNew:bool -> Matcher  (restart)
 Matcher  -> EvtSearchProgress -> Terminal (update info)
 Matcher  -> EvtSearchFin      -> Terminal (update list)
-Matcher  -> EvtHeader         -> Terminal (update header)
 */
 
 type revision struct {
@@ -113,14 +112,8 @@ func Run(opts *Options) (int, error) {
 	cache := NewChunkCache()
 	var chunkList *ChunkList
 	var itemIndex int32
-	header := make([]string, 0, opts.HeaderLines)
 	if opts.WithNth == nil {
 		chunkList = NewChunkList(cache, func(item *Item, data []byte) bool {
-			if len(header) < opts.HeaderLines {
-				header = append(header, byteString(data))
-				eventBox.Set(EvtHeader, header)
-				return false
-			}
 			item.text, item.colors = ansiProcessor(data)
 			item.text.Index = itemIndex
 			itemIndex++
@@ -147,11 +140,6 @@ func Run(opts *Options) (int, error) {
 				}
 			}
 			transformed := nthTransformer(tokens, itemIndex)
-			if len(header) < opts.HeaderLines {
-				header = append(header, transformed)
-				eventBox.Set(EvtHeader, header)
-				return false
-			}
 			item.text, item.colors = ansiProcessor(stringBytes(transformed))
 
 			// We should not trim trailing whitespaces with background colors
@@ -236,13 +224,15 @@ func Run(opts *Options) (int, error) {
 		denylist = make(map[int32]struct{})
 		denyMutex.Unlock()
 	}
+	headerLines := int32(opts.HeaderLines)
+	headerUpdated := false
 	patternBuilder := func(runes []rune) *Pattern {
 		denyMutex.Lock()
 		denylistCopy := maps.Clone(denylist)
 		denyMutex.Unlock()
 		return BuildPattern(cache, patternCache,
 			opts.Fuzzy, opts.FuzzyAlgo, opts.Extended, opts.Case, opts.Normalize, forward, withPos,
-			opts.Filter == nil, nth, opts.Delimiter, inputRevision, runes, denylistCopy)
+			opts.Filter == nil, nth, opts.Delimiter, inputRevision, runes, denylistCopy, headerLines)
 	}
 	matcher := NewMatcher(cache, patternBuilder, sort, opts.Tac, eventBox, inputRevision)
 
@@ -265,6 +255,9 @@ func Run(opts *Options) (int, error) {
 				func(runes []byte) bool {
 					item := Item{}
 					if chunkList.trans(&item, runes) {
+						if item.Index() < headerLines {
+							return false
+						}
 						mutex.Lock()
 						if result, _, _ := pattern.MatchItem(&item, false, slab); result != nil {
 							opts.Printer(transformer(&item))
@@ -349,11 +342,11 @@ func Run(opts *Options) (int, error) {
 			clearDenylist()
 		}
 		reading = true
+		headerUpdated = false
 		startTick = ticks
 		chunkList.Clear()
 		itemIndex = 0
 		inputRevision.bumpMajor()
-		header = make([]string, 0, opts.HeaderLines)
 		readyChan := make(chan bool)
 		go reader.restart(command, environ, readyChan)
 		<-readyChan
@@ -411,7 +404,11 @@ func Run(opts *Options) (int, error) {
 						snapshotRevision = inputRevision
 					}
 					total = count
-					terminal.UpdateCount(total, !reading, value.(*string))
+					terminal.UpdateCount(max(0, total-int(headerLines)), !reading, value.(*string))
+					if headerLines > 0 && !headerUpdated {
+						terminal.UpdateHeader(GetItems(snapshot, int(headerLines)))
+						headerUpdated = int32(total) >= headerLines
+					}
 					if heightUnknown && !deferred {
 						determine(!reading)
 					}
@@ -421,6 +418,7 @@ func Run(opts *Options) (int, error) {
 					var command *commandSpec
 					var environ []string
 					var changed bool
+					headerLinesChanged := false
 					switch val := value.(type) {
 					case searchRequest:
 						sort = val.sort
@@ -441,6 +439,12 @@ func Run(opts *Options) (int, error) {
 							nth = *val.nth
 							bump = true
 						}
+						if val.headerLines != nil {
+							headerLines = int32(*val.headerLines)
+							headerUpdated = false
+							headerLinesChanged = true
+							bump = true
+						}
 						if bump {
 							patternCache = make(map[string]*Pattern)
 							cache.Clear()
@@ -477,6 +481,14 @@ func Run(opts *Options) (int, error) {
 							snapshotRevision = inputRevision
 						}
 					}
+					if headerLinesChanged {
+						terminal.UpdateCount(max(0, total-int(headerLines)), !reading, nil)
+						if headerLines > 0 {
+							terminal.UpdateHeader(GetItems(snapshot, int(headerLines)))
+						} else {
+							terminal.UpdateHeader(nil)
+						}
+					}
 					matcher.Reset(snapshot, input(), true, !reading, sort, snapshotRevision)
 					delay = false
 
@@ -486,11 +498,6 @@ func Run(opts *Options) (int, error) {
 						terminal.UpdateProgress(val)
 					}
 
-				case EvtHeader:
-					headerPadded := make([]string, opts.HeaderLines)
-					copy(headerPadded, value.([]string))
-					terminal.UpdateHeader(headerPadded)
-
 				case EvtSearchFin:
 					switch val := value.(type) {
 					case MatchResult: