fix(color): degrade empty parent stack to transparent, chain fillers

Two follow-ups to 76ac76d/cdb4bfe8, found by an independent review:

- keywords.go's Resolve gated parent-keyword resolution on parents !=
  nil, not len(parents) != 0. terminal.String() (cdb4bfe) leaves
  ParentColors non-nil-but-empty after the very first block a process
  ever renders, so every parentBackground/parentForeground resolved
  against an empty stack after that point returned the literal keyword
  string instead of Transparent - which then fails color parsing and
  renders as no color at all, not even transparent.

- Engine.blockTailColors (76ac76d) captured a single *color.Set
  re-derived from the tail segment's own Resolve*() calls. A segment's
  stored color can itself be an unresolved parentBackground keyword
  (normal - full resolution happens later against the live stack), so
  a one-entry reseed had nowhere further to walk. Capture a full copy
  of terminal.ParentColors instead of re-deriving one entry, so a
  filler chains through an unresolved tail exactly like the block's
  own last segment did.

Entire-Checkpoint: 6582863c12cb

Author: JanDeDobbeleer

src/color/keywords.go

@@ -84,7 +84,7 @@ func (color Ansi) Resolve(current *Set, parents []*Set) Ansi {
 			return current.Background
 		case keyword == Foreground && current != nil:
 			return current.Foreground
-		case (keyword == ParentBackground || keyword == ParentForeground) && parents != nil:
+		case (keyword == ParentBackground || keyword == ParentForeground) && len(parents) != 0:
 			return resolveParentColor(keyword)
 		default:
 			return Transparent

src/prompt/engine.go

@@ -1,6 +1,7 @@
 package prompt
 
 import (
+	"slices"
 	"strings"
 	"sync"
 
@@ -25,13 +26,17 @@ type Engine struct {
 	Config                *config.Config
 	activeSegment         *config.Segment
 	previousActiveSegment *config.Segment
-	// blockTailColors is the last rendered segment's colors, captured just
+	// blockTailColors is a snapshot of terminal.ParentColors 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
+	// A full copy, not just the tail entry: the tail segment's own stored
+	// color can itself be an unresolved parentBackground/parentForeground
+	// keyword, and resolving that requires walking the rest of the chain
+	// the same way the block's own last segment did.
+	blockTailColors   []*color.Set
 	pendingSegments   sync.Map
 	rprompt           string
 	Overflow          config.Overflow
@@ -196,10 +201,10 @@ func (e *Engine) shouldFill(filler string, padLength int) (string, bool) {
 	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
+	// terminal.String() call produced blockText - reseed the chain a
 	// <parentBackground>/<parentForeground> anchor in the filler needs.
-	if e.blockTailColors != nil {
-		terminal.ParentColors = append(terminal.ParentColors, e.blockTailColors)
+	if len(e.blockTailColors) != 0 {
+		terminal.ParentColors = append(terminal.ParentColors, e.blockTailColors...)
 	}
 
 	terminal.Write("", "", filler)
@@ -518,21 +523,21 @@ 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
+// captureBlockTailColors snapshots terminal.ParentColors 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.
+// cleared the parent stack for the next block. A full copy of the chain, not
+// just the tail entry: the tail segment's own stored color can itself be an
+// unresolved parentBackground/parentForeground keyword, which needs the rest
+// of the chain to resolve the same way it did for the block's own segments.
 func (e *Engine) captureBlockTailColors() {
 	if e.previousActiveSegment == nil {
 		e.blockTailColors = nil
 		return
 	}
 
-	e.blockTailColors = &color.Set{
-		Background: e.previousActiveSegment.ResolveBackground(),
-		Foreground: e.previousActiveSegment.ResolveForeground(),
-	}
+	e.blockTailColors = slices.Clone(terminal.ParentColors)
 }
 
 func (e *Engine) writeSeparator(final bool) {