Cache output bundle previews in memory

[t-kalinowski]

Summary

This makes files-mode output bundle previews follow a one-way memory-to-disk model.

Previously, later replies for image-heavy output bundles could write preview images to disk and then read those same files back to rebuild inline previews. That made reply rendering depend on old bundle files still being present. This change keeps the bounded preview images in memory and treats the bundle directory as append-only history for clients and debugging.

User facing changes

No intended user-facing behavior change.

Output bundles still use the same public layout and disclosure paths: transcript.txt, events.log, images/, and images/history/.

Internal changes

• Cache the first image preview and latest image preview on ActiveOutputBundle.
• Stop rereading old bundle image files when rendering later inline previews.
• Update the plot regression so deleted bundle image files do not affect later inline previews.
• Add a text-bundle regression showing that a deleted transcript.txt is recreated for later appends without replaying earlier spilled text.
• Move the one-way output bundle plan to docs/plans/completed/.