Bolt-Era Highlight Collector Analysis

Lifted from `augment-it-bolt/specs/HighlightCollector-Analysis.md`. Per-feature breakdown of the highlight-collector module — the pipeline stage where users mark up LLM responses with section_title + color spans and persist them. One of the two stages that never got split into Tanuj's microfrontend repos, which means this document is the only existing description of how that stage works. Load-bearing reference for the rewrite.