bug(codex): a resumed rollout that starts with compacted hides its retained earlier prompts #1393

Found by review a of #1386.

Evidence

• In the local corpus, 82 rollout files (68 native sessions, 19 of them source: "cli") have a first compacted record that comes before every response_item and event_msg.
• In each of those files, replacement_history holds plain, non-bootstrap user text. That text is the only copy of the earlier conversation in the file.
• 58 of those sessions have no other rollout file locally.
• One concrete 0.149.0 CLI file: two session_meta records, then a compacted record holding 17 retained user messages, then world_state, turn_context and the later responses.

Current behaviour

• Before fix(codex): render committed compaction as a timestamped boundary #1386, no compacted line rendered at all: the payload-type guard returned [].
• After fix(codex): render committed compaction as a timestamped boundary #1386, the line renders as a boundary, plus the readable summary when there is one. replacement_history is still never mapped, because in the common case it only duplicates earlier prompts: 33,443 of 37,724 retained user items have an exact earlier match.
• So opening or previewing one of these resumed sessions shows only "Conversation compacted", with none of the 17 prompts.

Why the mapper cannot fix it alone

mapCodexRolloutToFeedEntries maps one line at a time. A paged older-history load can start a page with a compacted line whose predecessors sit in the previous page, so "first line in this mapping pass" is not the same as "first line in the file".

Direction

Where the history loader (initial, older-page and preview paths) knows it is mapping from file offset 0, map the retained user messages of a compacted record that no conversation record precedes. Mark them as retained history, and skip developer instructions and the encrypted compaction item. Add a recorded initial-compacted regression case, with fixtures redacted at the same length.