Fix nth attr merge order to respect precedence hierarchy (#4697)

* Fix nth attr merge order to respect precedence hierarchy

nth attrs were merged ON TOP of current-fg/selected-fg attrs, so
nth:regular would clear attrs like underline from current-fg. Fix the
merge chain to apply nth BEFORE the line-type overlay:

  fg < nth < selected-fg < current-fg < hl < selected-hl < current-hl

Store raw current-fg and selected-fg attrs in ColorTheme before they
get merged with fg/ListFg, then pass them as nthOverlay through
printHighlighted to colorOffsets where the correct merge chain is
computed: fgAttr.Merge(nthAttr).Merge(nthOverlay).

Fix #4687

* Make current-fg inherit from list-fg instead of fg

current-fg was inheriting from fg, not list-fg, so setting list-fg
had no effect on the current line. e.g.

  fzf --color fg:dim,list-fg:underline,nth:regular -d / --nth -1

The non-nth part of the current line was dim (from fg) instead of
underline (from list-fg). Resolve ListFg/ListBg earlier in InitTheme
so Current can inherit from them.

* Make selected-fg inherit from list-fg via merge instead of override

selected-fg used o() which replaces the attr from list-fg entirely.
e.g. with fg:dim,selected-fg:italic, the dim was lost on selected
lines because o() replaced dim with italic instead of merging them.
Use ColorAttr.Merge() so attrs are combined additively, consistent
with how current-fg inherits from list-fg.

* Apply selected-fg attrs on current line when item is selected

When an item is both current and selected, the current-line rendering
ignored selected-fg attrs entirely. e.g.

  echo foo | fzf --color fg:dim,nth:regular,current-fg:underline,selected-fg:italic --bind result:select --multi

The italic from selected-fg was lost. Merge NthSelectedAttr into the
overlay and rebuild colBase attr with the correct precedence chain:
fg < selected-fg < current-fg.

src/result.go

@@ -128,7 +128,7 @@ func minRank() Result {
 	return Result{item: &minItem, points: [4]uint16{math.MaxUint16, 0, 0, 0}}
 }
 
-func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, theme *tui.ColorTheme, colBase tui.ColorPair, colMatch tui.ColorPair, attrNth tui.Attr, hidden bool) []colorOffset {
+func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, theme *tui.ColorTheme, colBase tui.ColorPair, colMatch tui.ColorPair, attrNth tui.Attr, nthOverlay tui.Attr, hidden bool) []colorOffset {
 	itemColors := result.item.Colors()
 
 	// No ANSI codes
@@ -213,6 +213,10 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 		}
 		return tui.NewColorPair(fg, bg, ansi.color.attr).WithUl(ansi.color.ul).MergeAttr(base)
 	}
+	fgAttr := tui.ColNormal.Attr()
+	nthAttrFinal := fgAttr.Merge(attrNth).Merge(nthOverlay)
+	nthBase := colBase.WithNewAttr(nthAttrFinal)
+
 	var colors []colorOffset
 	add := func(idx int) {
 		if curr.fbg >= 0 {
@@ -226,7 +230,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 			if curr.match {
 				var color tui.ColorPair
 				if curr.nth {
-					color = colBase.WithAttr(attrNth).Merge(colMatch)
+					color = nthBase.Merge(colMatch)
 				} else {
 					color = colBase.Merge(colMatch)
 				}
@@ -246,7 +250,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 					if color.Fg().IsDefault() && origColor.HasBg() {
 						color = origColor
 						if curr.nth {
-							color = color.WithAttr(attrNth &^ tui.AttrRegular)
+							color = color.WithAttr((attrNth &^ tui.AttrRegular).Merge(nthOverlay))
 						}
 					} else {
 						color = origColor.MergeNonDefault(color)
@@ -258,7 +262,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 				ansi := itemColors[curr.index]
 				base := colBase
 				if curr.nth {
-					base = base.WithAttr(attrNth)
+					base = nthBase
 				}
 				if hidden {
 					base = base.WithFg(theme.Nomatch)
@@ -270,7 +274,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 					match:  false,
 					url:    ansi.color.url})
 			} else {
-				color := colBase.WithAttr(attrNth)
+				color := nthBase
 				if hidden {
 					color = color.WithFg(theme.Nomatch)
 				}