fix(prompt): resolve parentBackground in a block's own filler

cdb4bfe8 scoped terminal.ParentColors to one block, resetting it in
String() once a block's segments are done rendering. block.Filler (and
the transient prompt's filler) render after that reset - via
shouldFill, called from writeBlock with an already-built blockText -
so a filler template using <parentBackground>/<parentForeground>
started resolving against an empty stack instead of the block's last
segment.

Capture the last segment's colors into a new Engine.blockTailColors
field right before previousActiveSegment resets to nil (same spot in
both renderBlockFromCache and renderBlockSegments), and reseed
terminal.ParentColors with that one entry in shouldFill before writing
the filler. No stock theme uses filler with parent color keywords, so
this was unreachable in practice, but it's a real behavior change from
what shipped in cdb4bfe8.

Entire-Checkpoint: edb9d9774272

src/prompt/engine.go

@@ -25,16 +25,23 @@ type Engine struct {
 	Config                *config.Config
 	activeSegment         *config.Segment
 	previousActiveSegment *config.Segment
-	pendingSegments       sync.Map
-	rprompt               string
-	Overflow              config.Overflow
-	prompt                strings.Builder
-	allBlocks             []*config.Block
-	currentLineLength     int
-	Padding               int
-	rpromptLength         int
-	Plain                 bool
-	forceRender           bool
+	// blockTailColors is the last rendered segment's colors, captured just
+	// before previousActiveSegment resets to nil at the end of a block. A
+	// block's own Filler (see shouldFill) renders after terminal.String()
+	// has already cleared the parent stack for the next block, so a filler
+	// template using <parentBackground>/<parentForeground> needs this to
+	// resolve against the block it is padding rather than an empty stack.
+	blockTailColors   *color.Set
+	pendingSegments   sync.Map
+	rprompt           string
+	Overflow          config.Overflow
+	prompt            strings.Builder
+	allBlocks         []*config.Block
+	currentLineLength int
+	Padding           int
+	rpromptLength     int
+	Plain             bool
+	forceRender       bool
 }
 
 const (
@@ -187,6 +194,14 @@ func (e *Engine) shouldFill(filler string, padLength int) (string, bool) {
 
 	// allow for easy color overrides and templates
 	terminal.SetColors("default", "default")
+
+	// the block's own segments already reset the parent stack when their
+	// terminal.String() call produced blockText - reseed the one entry a
+	// <parentBackground>/<parentForeground> anchor in the filler needs.
+	if e.blockTailColors != nil {
+		terminal.ParentColors = append(terminal.ParentColors, e.blockTailColors)
+	}
+
 	terminal.Write("", "", filler)
 	filler, lenFiller := terminal.String()
 	if lenFiller == 0 {
@@ -337,6 +352,7 @@ func (e *Engine) renderBlockFromCache(block *config.Block, cancelNewline bool) b
 
 	e.writeSeparator(true)
 	e.activeSegment = nil
+	e.captureBlockTailColors()
 	e.previousActiveSegment = nil
 
 	blockText, length := terminal.String()
@@ -502,6 +518,23 @@ func (e *Engine) renderActiveSegment() {
 	terminal.SetParentColors(e.previousActiveSegment.ResolveBackground(), e.previousActiveSegment.ResolveForeground())
 }
 
+// captureBlockTailColors snapshots the last rendered segment's colors into
+// blockTailColors just before previousActiveSegment resets to nil, so a
+// later shouldFill call for this same block's Filler can still resolve
+// <parentBackground>/<parentForeground> after terminal.String() has already
+// cleared the parent stack for the next block.
+func (e *Engine) captureBlockTailColors() {
+	if e.previousActiveSegment == nil {
+		e.blockTailColors = nil
+		return
+	}
+
+	e.blockTailColors = &color.Set{
+		Background: e.previousActiveSegment.ResolveBackground(),
+		Foreground: e.previousActiveSegment.ResolveForeground(),
+	}
+}
+
 func (e *Engine) writeSeparator(final bool) {
 	if e.activeSegment == nil {
 		return

src/prompt/segments.go

@@ -80,6 +80,7 @@ func (e *Engine) renderBlockSegments(results []*config.Segment, block *config.Bl
 	e.writeSeparator(true)
 
 	e.activeSegment = nil
+	e.captureBlockTailColors()
 	e.previousActiveSegment = nil
 
 	return terminal.String()