fix(snapshot): handle stale dedup references and empty baselines

[bobleer]

Summary

• Only treat stored content as a deduplicated Reference when the backing content file still exists (avoids stale hash map entries after disk changes).
• When building snapshot rows, use empty compressed bytes for Reference content instead of unreachable! (matches empty restore path).
• Add an async core test that creates an empty baseline, snapshots an empty file, and asserts restore is empty without panicking.
• Rustfmt-only touch-ups in agentic modules and the Anthropic stream processor test.

Testing

• cargo test -p bitfun-core