fix(terminal): stop parent color stack growing unbounded

ParentColors accumulated every segment ever rendered across the whole
process lifetime, never reset - harmless for a one-shot render but
quadratic in the pwsh serve daemon, where StreamPrimary keeps calling
Primary() in the same long-lived process. Each render's SetParentColors
also copied the full history on every call, so cost compounded further.

Push onto a real stack instead of prepend-copying, and clear it once
per block in String() where the rest of the per-block render state
already resets. resolveParentColor's walk order flips to match: nearest
ancestor is now the tail, not the head.

BenchmarkEnginePrimary at 64000 iterations: 670µs/op -> 16µs/op, flat
instead of growing with iteration count; SetParentColors accounted for
99% of all allocated bytes in a mem profile before this change.

Entire-Checkpoint: 7b56670e64a2

src/color/keywords.go

@@ -26,7 +26,10 @@ func (color Ansi) isKeyword() bool {
 
 func (color Ansi) Resolve(current *Set, parents []*Set) Ansi {
 	resolveParentColor := func(keyword Ansi) Ansi {
-		for _, parentColor := range parents {
+		// parents is a stack pushed tail-first (see terminal.SetParentColors):
+		// the nearest ancestor is the last element, so walk back-to-front.
+		for i := len(parents) - 1; i >= 0; i-- {
+			parentColor := parents[i]
 			if parentColor == nil {
 				return Transparent
 			}

src/terminal/writer.go

@@ -218,15 +218,15 @@ func SetColors(background, foreground color.Ansi) {
 	}
 }
 
+// SetParentColors pushes the completed segment's colors onto the parent
+// stack; the most recent entry (nearest ancestor) sits at the tail. Cleared
+// per block by String() - see resolveParentColor in color/keywords.go for
+// the matching tail-to-head walk.
 func SetParentColors(background, foreground color.Ansi) {
-	if ParentColors == nil {
-		ParentColors = make([]*color.Set, 0)
-	}
-
-	ParentColors = append([]*color.Set{{
+	ParentColors = append(ParentColors, &color.Set{
 		Background: background,
 		Foreground: foreground,
-	}}, ParentColors...)
+	})
 }
 
 func ChangeLine(numberOfLines int) string {
@@ -602,6 +602,11 @@ func String() (string, int) {
 
 		bgGradientCells, fgGradientCells = nil, nil
 		cellIndex = 0
+
+		// the parent stack is scoped to one block; each new block starts a
+		// fresh ancestor chain. Slicing to zero keeps the backing array so
+		// same-size blocks (the common case) push without reallocating.
+		ParentColors = ParentColors[:0]
 	}()
 
 	return builder.String(), length