101efd13

Buffer the shard supervisor's JSON reads

`read_json` handed an unbuffered `File` to `serde_json::from_reader`, one
syscall per byte. Harmless for the small snapshot/state files, but the
supervisor now reads walk manifests that legitimately reach megabytes; the
padded fake-child manifest made the pause test take minutes on a loaded
machine (suite 246 s instead of 18 s). A `BufReader` restores the cost.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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