The cache dropped EVERY entry once it passed 20 000. That was survivable while
the only feed was this station's own WSJT-X decodes, which never reached the
ceiling — it is a cliff for anything larger, and every locator in the cluster
list would disappear at once, periodically, for no reason the operator could
see.
Two generations: when the current map fills it becomes the previous one and a
fresh map takes over; lookups consult both. A rotation therefore costs the
older half and nothing more. It needs no insertion order, no per-entry
timestamp and no bookkeeping on the write path — all of which "evict the
oldest thousand" would require, on a path that runs once per decode.
The cap is 100 000, measured at 82 bytes an entry: 8 MB a generation, 16 MB
for both. 20 000 was chosen when the ceiling was unreachable anyway.
Both maps now go through rememberDecodeGrid / lookupDecodeGrid. Rotation swaps
the map headers, so the read had to be guarded; routing every access through
one accessor is what makes that checkable rather than remembered.
