perf(web): reuse timeline rows while text streams (#9725)

Reuse ordered timeline entries and raw rows for safe streaming text updates. Keep full derivation for structural, metadata, activity, and control changes.

Preserve immutable attachment-preview objects when their current URLs are unchanged. Cover URL renewal/removal, completion, grouping, pagination, and earlier-row immutability.

Continue extoci's ordered timeline projection work with separate integration and attachment corrections. The original contribution remains unchanged.

Created with GPT-6 Astra (preview) in Codex.

Co-Authored-By: extoci <hi@extoci.lol>