Chunk objects, shard the store, and make filler configurable

File bodies are now cut into 4 MiB objects, the last padded to its
power-of-two bucket, so every object is 64 KiB to 4 MiB whatever the file
size. Object IDs are HMAC'd from a secret per-record seed, so nothing on
disk links a file's pieces, and each object still leads with its own ID,
binding it to its record and position. Chunks are encrypted and synced by
a small worker pool so their fsyncs overlap.

records/ and objects/ are sharded into 256 subdirectories by ID prefix,
all created at init, with every shard's mtime pinned to the epoch.

vault init --filler SIZE writes at least SIZE of filler. Filler entries
are now real entries for a throwaway key, of log-uniform random sizes, so
they carry multi-chunk files like a real collection.

The marker becomes VAULT 2; earlier vaults are not supported.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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