weichen
8e2657c824
Why: the delivery log for the 2026-09-30 afternoon fixes (part-order fidelity, download channel, artifact file management, settings cascade) belongs with the redesign proposal it extends, including the two durable conventions it produced. Evidence: adds section 9 with root causes, the render-order-equals- storage-order rule, the compound-selector lesson for @ui shells on host generic classes, and the full verification record (vitest/go suites plus the 12/12 headless run). Impact: future UI work inherits both conventions without rediscovering the failure modes. Refs: task feature/chat-order-artifact-files.