Keep two live Codex rollout files with one session id from superseding each other #4328

Found by the #4279 lane (PR #4326). Two live rollout files that share one Codex session id share one observation source identity. Ingest reads them as rewrites of each other, so the later file's messages supersede the earlier file's, dropping messages.

Acceptance: reproduce with two synthetic rollout files carrying the same session id; give each file its own source identity (or merge them deliberately, with the reason documented); show both files' messages project and a rewrite of one file still supersedes only its own earlier content.