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

IMPLEMENTATION_NOTES.md

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+# Implementation Notes — Issue #23
+
+**Issue:** Add upgrade entrypoint to farming-pool for WASM hot-swap
+**Upstream:** https://github.com/SmartDropLabs/smartdrop-contracts/issues/23
+
+## 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:
+
+```rust
+/// 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 before calling this function.
+/// Emits a `("pool", "upgraded")` event with the new hash.
+pub fn upgrade(env: Env, new_wasm_hash: BytesN<32>) -> Result<(), PoolError> {
+    get_admin(&env).require_auth();
+    bump_instance(&env);
+    env.deployer().update_current_contract_wasm(new_wasm_hash.clone());
+    env.events().publish(
+        (symbol_short!("pool"), symbol_short!("upgraded")),
+        new_wasm_hash,
+    );
+    Ok(())
+}
+```
+
+Also add a corresponding `upgrade_pool(pool_id, new_wasm_hash)` to the factory that:
+1. Verifies the caller is the factory admin
+2. Calls the pool's `upgrade` function via cross-contract invocation
+3. Does NOT update the factory's stored `WasmHash` (pool-by-pool upgrade, not factory-wide)
+
+## Migration Safety
+
+The upgrade process requires that the new WASM's storage schema is backwards-compatible with existing `DataKey` entries. Add a `SCHEMA_VERSION` constant a
+
+---
+_Delete this file before merging._