[WIP] Add upgrade entrypoint to farming-pool for WASM hot-swap

[gaiabio12-design]

Closes #23

Proposed Approach

I've been looking at this issue — the lack of in-place WASM upgrade on FarmingPool is a real gap since Soroban's env.deployer().update_current_contract_wasm() makes this totally doable without losing staker state.

Here's how I'd approach it:

1. Add the upgrade function to contracts/soroban/farming-pool/src/lib.rs with admin-only guard using the existing auth pattern, emit an upgraded event with the new hash, and wire in the SCHEMA_VERSION constant for versioning.

2. Mirror this on Factory with upgrade_pool that's factory-admin-only and enforces the same error handling (PoolError::Unauthorized for non-admins).

3. Write two integration tests in the test suite — one verifying that after upgrade, all Position and Stake storage is preserved and balances match, another confirming the new contract functions are callable post-upgrade.

4. Make sure the upgrade path doesn't touch the existing position/stake structs; I'll keep the storage layout stable so migrations aren't needed.

The repo's got 58 files in soroban/ so there's precedent for these patterns. Can have a PR ready tomorrow.

Acceptance Criteria

Overview

Soroban contracts can upgrade their own WASM at runtime via env.deployer().update_current_contract_wasm(new_hash). The farming-pool contract has no such entrypoint, making bug fixes require deploying an entirely new pool — all staker positions, credits, and lock states are lost.

Design

Add an admin-only upgrade function that replaces the contract WASM in place:

/// Admin: replace this contract's WASM with the implementation at `new_wasm_hash`.
///
/// Storage (positions, stakes, credits) is preserved across upgrades.
/// The new WASM must be installed on-chain be

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_Implementation in progress — will remove `[WIP]` once code is ready._