Reasoning-heavy sessions accumulate 250-480KB of anchor_activity_scenes serialized
BEFORE the messages array, so the 64KB metadata-prefix read always overflowed ->
full-parse fallback on every read -> memory growth. Move scene data AFTER messages;
metadata prefix now ~2KB, modern reads skip the full parse. Old-layout files still
load (one authoritative parse then cached); round-trip preserves scenes exactly.
Independent review: nesquena APPROVED (end-to-end, on head). Gate-pass: Codex SAFE
(933KB legacy sidecar loads clean + round-trip verified), 17 own + 2945 regression green.
Co-authored-by: t <a@b>
Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>