add todo regarding cache ttl on old merge data

TODO.md

@@ -452,6 +452,45 @@ Eastern rather than a rolling window. This would avoid unnecessary refetches
 during the trading day and ensure a fetch shortly after close gets fresh data.
 Probably alleviated by the cron job approach.
 
+## Cache TTL semantics on merge writes — priority LOW
+
+The `writeMerged` primitive in `cache/store.zig` rewrites `dividends.srf` /
+`splits.srf` with `expires = now + ttl` whenever it adds a new record or
+upgrades fields on an existing one. This is conceptually wrong: TTL should
+reflect "when do we expect new information from the primary provider?",
+which is a property of the conversation with that provider — not of the
+file's last-modification time. Adding a 25-year-old historical dividend
+that Tiingo just supplied tells us nothing about Polygon's freshness; we
+shouldn't bump the file's expiry as a side effect.
+
+The cleaner design:
+
+- Cache file's `#!expires=` reflects "when did Polygon (the primary) last
+  say `here's everything I have`?"
+- Tiingo merge writes preserve the existing expires, only rewriting records.
+- Only `fetchCached`'s post-Polygon-fetch write bumps expires.
+
+In practice the current behavior caused exactly one observable problem: a
+one-time TTL herd on 2026-06-04 when the new merge code's first run added
+pre-2010 Tiingo backfill across 23+ symbols in a single overnight burst,
+and they all inherited that day's clock for `expires = now + 14d`. We
+manually re-staggered (`stagger_cache_ttls.py`) and moved on.
+
+Steady-state risk: minimal. The merge primitive's "skip if nothing changed"
+branch means no-op refreshes don't bump expires. New entries from genuinely
+new dividends are spread across the calendar by the dividends themselves
+(quarterly cadence varies per ticker). Field upgrades stop firing once
+Polygon's metadata is in place.
+
+When this could matter again:
+- Adding a third source for div/splits (TTL semantics get murkier).
+- Wiping and rebuilding the server cache (one-time herd recurs).
+- A long pause in nightly refreshes followed by a backlog of merge writes.
+
+Fix would be small: thread `?expires_override` into `writeMerged` and have
+the merge path call `serializeWithMeta` with the existing expires (from the
+read) when source_hint isn't the primary.
+
 ## On-demand server-side fetch for new symbols
 
 Currently the server's SRF endpoints (`/candles`, `/dividends`, etc.) are pure