feature(pool): add SEP-41 approve()/allowance()/transfer_from() for LP shares

[K1NGD4VID]

Summary

Full SEP-41 compliance requires the allowance half of the interface (approve/allowance/transfer_from), not just direct transfer — needed for LP shares to compose with contracts that need pre-authorized spending (e.g. a future DEX listing or the LP-governance staking design from round 2's issue #718).

Acceptance Criteria

• approve(env: Env, from: Address, spender: Address, amount: i128, expiration_ledger: u32) added to contracts/pool/src/lib.rs, storing a new DataKey::Allowance(Address, Address) entry with expiry
• allowance(env: Env, from: Address, spender: Address) -> i128 added, returning 0 for an expired or unset allowance
• transfer_from(env: Env, spender: Address, from: Address, to: Address, amount: i128) added, requiring spender.require_auth() and decrementing the stored allowance
• Test added/updated and passing (cargo test), covering approve → transfer_from → allowance decremented, and transfer_from failing once the allowance is exhausted

Tech Stack

Rust, Soroban SDK, contracts/pool/src/lib.rs, contracts/pool/src/types.rs

[Crystalypto]

@Crystalypto has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

Hi , please assign this to me

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[mallison031]

@mallison031 has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

I will like to work on this

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