KIP-178 — Size-based log directory selection strategy

Discussion Broker

Adds a broker-side log directory selection strategy setting with two modes: `partition`, the existing heuristic that picks the `log.dirs` entry holding the fewest partitions, and `size`, which reads real free space through Java `File.getFreeSpace()` and picks the emptiest directory, falling back to partition counts when a `SecurityException` is raised. Partition-count placement distributes data poorly when partition volumes are uneven, so in production some log directories run nearly full while others keep large amounts of unused space.

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