Add completed group cloning

[er1c-cartman]

Summary:

• Adds clone_group(admin, source_group_id) -> u64 for completed groups.
• Copies token, contribution amount, cycle length, max members, and previous members into a new forming group.
• Stores source_group_id metadata on groups and registers cloned membership for previous members.
• Adds regression coverage showing a completed group can be cloned and the clone can start a new cycle.

Fixes #70.

Validation:

• cargo test --package sorosave
• cargo fmt --check
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Notes:

• Generated build/test artifacts were not committed.
• No secrets, tokens, or private runtime data are included.