Module watermark 

Per-project “last session start” watermark.

Stored at ~/.difflore/projects/{hash}/last-session-start.json as a one-shot JSON blob { "ts_ms": …, "client": "…" }. Owning this file is a single-purpose responsibility — no other code reads or writes it — so the format can change freely as long as the read path stays permissive about missing/malformed input (silent fallback to None, never panics).

Concurrent SessionStart fires from two agent windows in the same repo could race the write here. That’s fine: whichever fires last wins, and the only consequence is one of the two banners may show a slightly older prev_ts. We deliberately do NOT take a lock — the whole helper is on the hot path and a contended file lock would defeat the 50 ms budget.

Structs

Watermark

One row in the watermark file. client is purely diagnostic — the banner pipeline doesn’t branch on it today, but storing it lets a future audit answer “which agent last opened this repo?” without crawling the fire log.

Functions

read_watermark

Read the watermark for the given project hash. Returns None when:

write_watermark

Write the watermark for the given project hash. Best-effort: caller can ignore the Result via let _ = …. Creates the parent dir on demand — first-ever SessionStart in a repo finds ~/.difflore/projects/{hash}/ missing.