Assess composer-to-message attachment continuity

Assess shared-element attachment handoff from composer to optimistic message. Only build if identity, scrolling/virtualization, final geometry and pending/failure semantics can be preserved with a small change. Otherwise record evidence for deferral.

Requested by Alex after the T3 UI motion audit. First record an independent Astra-medium accept/revise/reject verdict with source evidence. Accepted changes should end in small focused PRs; do not implement a rejected suggestion.

Use the installed shadcn-motion-ui skill and Human Interface Craft guidelines (feedback, relationship, continuity, accessibility, signature moments). Consult current official docs, reuse local primitives, verify installed API compatibility and add exact source references. Respect native platforms, reduced motion, interruptions, responsiveness, focus, and performance. No continuous repaint loops or framework rewrites.

Acceptance:

• Independent assessment and overlap check recorded.
• Accepted scope implemented and focused checks pass, or rejection/dependency documented.
• Best-effort independent cross-provider review recorded.
• Real before/after motion evidence and PR linked, with any missing evidence disclosed as a draft-readiness gap.

Do not change live user data, the dirty primary checkout, existing Stash worktree, or unrelated work. No merge or deployment is requested.

Start by tracing the composer-to-optimistic-message flow and reviewing the installed shadcn-motion-ui skill, Human Interface Craft guidance, and current official API docs. Done means an independent verdict with overlap and source evidence, plus either a focused implementation with checks and before/after motion evidence or documented rejection/dependency and missing evidence.