fix(ai-chat): cache markdown parse, equatable code blocks, native scrollPosition (#1239)

[datlechin]

Closes #1239.

Depends on #1243 (build fix for main). Merge that first.

Symptoms

• CPU pegs at 100% when scrolling in AI chat that has any non-trivial response.
• Chat appears empty mid-stream / on completion; clicking the chevron-down brings content back.

Root cause

Three compounding bugs in MarkdownView.swift + AIChatPanelView.swift:

1. MarkdownBlock.id = UUID() (and MarkdownListItem.id = UUID()). Every parse generated fresh UUIDs, so ForEach thought every block was new. SwiftUI tore down + rebuilt all child views on every re-render — including AIChatCodeBlockView, which instantiates a full CodeEditSourceEditor (tree-sitter, theme, layout) per block.
2. MarkdownView.body re-parsed every evaluation. MarkdownBlockParser.parse(source) ran inline as a let in body. Combined with bug 1, every scroll frame and every streaming chunk re-parsed every visible message and recreated every code block.
3. Bottom-anchor scroll detection. A 1-px Color.clear at the end of the LazyVStack toggled isUserScrolledUp via onAppear/onDisappear. When the heavy re-render churn (from bugs 1+2) caused cells to flicker out of materialization, the chevron overlaid an apparently-empty chat. The user pressing the chevron forced a layout pass that re-materialized cells.

For a 5000-character response with 3 code blocks, each scroll frame was re-parsing ~100 markdown blocks and recreating 3 tree-sitter editors. That's the 100% CPU.

Fix

Four changes, no quick wins, native macOS APIs throughout:

MarkdownView.swift

• Stable identity: MarkdownBlock.id and MarkdownListItem.id switched from UUID() to deterministic Int indexes (top-level position, item-index within list). SwiftUI now preserves view identity across re-parses.
• Parse cache: new MarkdownDocumentCache stored as @State per MarkdownView instance. Identical source returns cached blocks, no re-parse.
• Inline-parse cache: MarkdownInline.parse backed by an NSCache<NSString, NSAttributedString> (bounded at 4000 entries). Repeated inline parses (table cells, list items) are now O(1) lookups.
• Equatable everywhere: MarkdownBlock, MarkdownBlock.Kind, MarkdownListItem, MarkdownTableAlignment all conform. MarkdownBlockView adopts Equatable + .equatable() so SwiftUI skips re-render when the block is unchanged.

AIChatCodeBlockView.swift

• Added Equatable conformance keyed on (code, language). The SourceEditor is no longer recreated when only ancestor state changes.

AIChatPanelView.swift

• Replaced the Color.clear bottom anchor + onAppear/onDisappear toggle with the native macOS 14+ .scrollPosition(id:anchor: .bottom) API.
• New state model: bottomVisibleMessageID (driven by SwiftUI) + pinnedToBottom (derived: true when the bottommost visible message is the last message). The chevron-down button shows only when pinnedToBottom is false.
• Programmatic auto-scroll-to-latest writes bottomVisibleMessageID = lastMessageID instead of using ScrollViewProxy.scrollTo("bottomAnchor"). ScrollViewReader removed.
• .scrollTargetLayout() added so SwiftUI can resolve target IDs.

Why this is the right architecture

• Identity is content-addressed, not allocation-addressed. SwiftUI diffing works correctly across re-renders.
• Parsing happens once per unique source string, not once per body call.
• SourceEditor lifecycle is decoupled from parent re-renders via Equatable. The expensive tree-sitter setup only runs when the underlying code actually changes.
• Scroll detection is one native API call instead of a fragile geometry-driven side effect. Survives content reflow during streaming.

Test plan

• Send the AI a prompt that yields ~3000 chars of mixed markdown + 2 code blocks. Scroll up/down rapidly. CPU should stay reasonable; chat content stays visible the whole time.
• Stream completion (text → markdown transition) no longer flashes blank.
• Scroll up; chevron-down appears. Click it; smooth scroll to bottom.
• Scroll back to bottom manually; chevron disappears, auto-scroll resumes for new messages.
• Switch between conversations; auto-scroll to bottom of new conversation.

Files changed

• Views/AIChat/MarkdownView.swift (+~140 / -~100, structural rewrite)
• Views/AIChat/AIChatCodeBlockView.swift (+5)
• Views/AIChat/AIChatPanelView.swift (+~30 / -~30 messageList rewrite, scrollToBottom helper deleted)
• CHANGELOG.md (+1)