R3: Render IM collaboration evidence blocks

[DeliciousBuding]

Summary

• adds IM collaboration evidence rendering for child agent, route decision, artifact, and deploy blocks
• keeps the transcript contract focused on visible evidence blocks instead of introducing another message stream

Validation

• corepack.cmd pnpm exec vitest run src/__tests__/IMBlockRenderer.test.tsx src/__tests__/IMMessageView.test.tsx (31/31)
• git diff --check

Merge notes

Draft until merge ordering is settled with R6A, which also touches IM message rendering/tests. Broad pnpm typecheck is still blocked by pre-existing Desktop type debt.

[DeliciousBuding]

Superseded by #278, which integrates R3/R5/R6A into one Desktop P0 PR.