feat: implement Soroban pool ownership contract logic

[Chucks1093]

Closes #11

What was implemented

Pool units / shares support

• Added total_units: u64 field to the Pool struct to represent the total number of ownership shares available in the pool (e.g. 100 units = 100 investable shares).
• create_pool now accepts total_units as a required parameter and validates it is non-zero.

Oversubscription prevention

• record_investment now checks whether total_invested + amount would exceed target_amount before accepting the investment.
• Returns ContractError::Oversubscribed (variant 5) if the investment would push the pool past its funding cap.
• Added ContractError::PoolInactive (variant 6) — record_investment also rejects investments into a closed pool.

Investor share calculation

• Added get_investor_share(investor, pool_id) -> u64 which returns the investor's ownership stake in basis points (bps): invested * 10_000 / target_amount. A 25% stake returns 2500.

Pool lifecycle: close

• Added close_pool(owner, pool_id) which requires owner auth and sets pool.active = false, preventing further investment without deleting pool data.

Tests added (test.rs)

All existing tests were updated to pass total_units to create_pool. New tests added:

Test	What it verifies
rejects_oversubscription	An investment that would exceed target_amount is rejected; exact-remainder investment succeeds
updates_duplicate_investor_position	Two investments by the same investor accumulate correctly on both the position and pool totals
calculates_investor_share_in_bps	A 2 500 / 10 000 investment returns exactly 2 500 bps
rejects_investment_into_closed_pool	After close_pool, record_investment returns PoolInactive