Add support for stable hash-based session deduplication in snapshot storage

[pugsley76]

Description:
Snapshot storage should deduplicate identical sessions using stable hashing to save disk space.

Work to be done:
Implement content-addressed storage for snapshots with deduplication.

Implementation procedure:

1. Add stable hashing for session snapshot content.
2. Store snapshots by hash and avoid duplicate copies.
3. Add tests for deduplication and recovery.
4. Document snapshot storage semantics.

Acceptance criteria:

• Snapshot storage deduplicates identical sessions.
• Hash-based storage is reliable.
• Tests cover deduplication behavior.
• Documentation explains storage semantics.

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