LogStore::new folded `(now - last_clock)` into every entry's age_ms, but
age_ms is persisted at INSERT time and stored_at was never caught up at
flush. So an observation inserted before the last flush lost the
`last_flush - insert_time` after restart (a store that was non-idle
between insert and flush reported an age too small by that gap).
Fix by anchoring each entry's age to its own insert tick: carry
`stored_at` on the Obs record and fold `(now - stored_at)` per-entry at
load, instead of a single global fold against the last clock marker.
flush() now also advances each in-memory age to its current value so the
representation stays coherent within a run.
This is exact for idle stores and observations inserted before the last
flush. Adds tests/persistence.rs::ages_rebase_after_nonidle_flush
(insert A at tick 1000, flush at 3000, reopen at 9000 -> asserts true
age 8100, not 6100).
kyle
ef53204fca