Codex reproduced a SILENT regression: the PR's 4096-BYTE read could miss the
continuation marker the old read_text()[:4096] (4096 CHARS) found. Session.save()
writes compression_anchor_summary (ensure_ascii=False) BEFORE parent_session_id,
so a multi-byte emoji summary puts the marker at char offset ~1230 but byte
offset ~4900 — within the old 4096-char window, past a 4096-byte one → continuation
child no longer detected. Fix: read 16384 bytes (4096 chars worst-case for 4-byte
UTF-8) then slice [:4096] chars, keeping the same needle + bounded I/O. Multibyte
regression test added + proven non-vacuous; updated the read-size assertion to <=16384.
Co-authored-by: starship-s <starship-s@users.noreply.github.com>
