The hard-coded 500 MB per-entry cap meant operators who set a tighter
files.maxsize could not actually enforce it on imports. Derive the cap
from files.maxsize with a floor so data.json / filters.json / VERSION
entries can still be read when the configured limit is tiny.
Clamp the uint64->int64 conversion and the LimitReader cap so absurd
configuration values do not overflow into MinInt64 and cause
io.LimitReader to treat every entry as EOF.
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