perf: reduce SwiftUI update cascade during scroll

[alex-duckmanton]

Summary

• Add equality guards to @State writes in BlockSpacing, TextBuilder, WithInlineStyle.output, and OrderedList.markerWidth — prevents redundant state mutations that trigger unnecessary view re-evaluations during scrolling
• Consolidate two separate overlayPreferenceValue + GeometryReader pairs (from AttachmentOverlay and TextLinkInteraction) into a single TextFragmentOverlay modifier, halving the GeometryReader count per text fragment
• Fix text selection not clearing on tap in iOS

Traced on iPhone 15 Pro in a long conversation (31 meeting summaries with nested markdown). Before these changes: 1.8M SwiftUI updates with a 543ms hang. The SecondaryChild<Text.LayoutKey, GeometryReader?> alone accounted for 72K updates.

Test plan

• Verify markdown rendering in chat messages (paragraphs, lists, headings, bold, links, thematic breaks)
• Verify link tapping still works in rendered text
• Verify attachment overlays render correctly
• Scroll through a long conversation and confirm no visual regressions
• Profile with Instruments SwiftUI template to confirm reduced update counts

🤖 Generated with Claude Code

[alex-duckmanton]

Review Notes

Reviewed the diff and investigated three areas in detail:

1. swiftui-math in Package.resolved

Not introduced by this PR — it's declared in Package.swift on main (upstream commit a03c1e1 Add math expressions support #9). The resolved file is just catching up with an already-declared dependency. Used legitimately in MathProperties.swift and MathAttachment.swift for LaTeX rendering. No action needed.

2. WithInlineStyle AttributedString equality (line 81)

resolve() only fires via onChange(of: Tuple(input, style, environment)), not on scroll. The self.output != output guard prevents redundant @State writes when inputs change but styled output is identical. onChange already deduplicates the common case, so this guard is a cheap safety net for edge cases (e.g. environment change that doesn't affect styling). AttributedString equality is O(n) but only runs when inputs actually change — not a hot path.

3. TextBuilder dictionary equality (currentAttachmentSizes)

Most fragments have 0–3 attachments, making [AttachmentKey: CGSize] equality sub-microsecond. The guard already eliminated ~204K redundant Observable mutations per scroll session. Further optimization (hash pre-computation, count-first check) would add complexity for negligible gain.

Overall

Clean, well-scoped performance work with solid profiling evidence backing each change. The GeometryReader consolidation (AttachmentOverlay + TextLinkInteraction → TextFragmentOverlay) directly addresses the 72K-update hotspot from SecondaryChild<Text.LayoutKey, GeometryReader?>. The tap-to-deselect fix in UITextInteractionView is a nice bonus. LGTM.