Preserve styles in tracked composite updates

[Aaronontheweb]

Parent: #366

Source

• Netclaw OpenSpec design
• Issue map

Producer and consumer

• Producer: Tracked composite segments
• Consumer: Applications with live styled rows

Scope

• Preserve each child style after keyed replacement.
• Preserve block boundaries after replacement.
• Keep the current tracked segment API compatible.

Acceptance

Positive cases:

• Mixed colors and decorations survive replacement.
• A block replacement remains a separate block.

Negative cases:

• One child style does not leak into a sibling.
• An inline segment does not lose an explicit block boundary.

Proof

• Add mixed-style regression tests.
• Add multiline block replacement tests.

Prerequisites

• None.