Add get_shares(id) returning just the shares vector

[BernardOnuh]

Closes #104

Adds get_shares(id) -> Result<Vec, Error> that returns only the shares vector without loading the full Split struct.

• New function get_shares in lib.rs:595-597
• Two tests: get_shares_returns_only_shares and get_shares_panics_on_missing_split
• All 57 tests passing
• cargo fmt and cargo clippy --all-targets -- -D warnings clean

[Spagero763]

@BernardOnuh kindly resolve conflictrs