Cap view distance at what the clients actually render

Nobody sees further than their own client draws. If the highest client setting in a world is 8, a server view distance of 16 loads, ticks and pays for rings of chunks that are never sent to anyone.

The new client-view-distance-cap section caps a world's view distance at the largest value its players' clients report, plus a small headroom. It never goes below that world's min-view-distance, simulation distance follows so view >= sim still holds, and an empty world is left alone. A client that never reported a value counts as unknown rather than as a cap of zero.

Off by default. It only pays on servers whose players run low settings, and changes nothing where everyone uses 16 or more. /vdt status shows the highest client value per world when it is on.

A config upgrade that quietly did nothing

Found while building the above, and it would have affected every future option: the upgrader keeps a hand-maintained list of the sections it knows. A section missing from that list was never added to an existing config, while the config version marker was raised anyway, after which the upgrader stayed silent about it for good. The option then ran on its code default and nobody could find it in their file.

There is now a test comparing that list against the shipped config, so a new section cannot slip through again.

Shorter comments

Roughly 1700 lines of explanatory comment came out of the source. The reasoning behind each fix lives in the commit that made it; what stays in the code is the short note that keeps a line from being rewritten wrongly. No behaviour changed.

Upgrading

Replace the jar and restart. The new section is added to your existing config.yml automatically, switched off.