log: cap debug.log in-process, and ship a logrotate policy

debug.log grew without bound (production: 1.95 GiB, 51-129 MB/day).
Nothing shrank it once the process was up -- there is no
ShrinkDebugFile in this tree, and upstream's only ran at startup.

OutputDebugStringF now rolls the file to debug.log.old when it
passes -maxdebugfilesize (MB, default 1024, 0 disables). The write
path already opens and closes the file per line under one lock, so
the rename-and-reopen is a few lines inside that lock and no fd can
be stranded mid-roll.

contrib/logrotate/bitflash-debug.logrotate is the longer-term policy
(size 1G, rotate 4, compress, missingok, notifempty, copytruncate);
copytruncate keeps the writer's append handle valid across rotation.

Closes #34

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