Expiry is now ruled out: 36/36 sampled images from 2025-09 through
2026-08 are still live server-side, so nothing dies of age and export
cadence is not the variable. The loss is per-asset — 2026-07-09 kept 38
images and lost 12 in one conversation.
Next hypothesis: those images belong to messages that were edited or
regenerated. ChatGPT stores a conversation as a tree and editing forks it,
stranding the superseded messages on an abandoned branch. The exporter
walks every node (chatgpt.py:963), so it exports those branches too — and
an attachment unreachable from live history is a natural GC target.
branch_check.py fetches the raw conversation, derives the live branch by
walking parent links up from current_node, and cross-tabulates every image
against (on the live branch?) x (still downloadable?). If the lost images
are all off-branch and nothing on the live branch is missing, this is not
data loss at all — it is attachments to messages that were replaced.
