feat: Yield Strategy Interface, Registry, and Deposit Routing

[OtowoSamuel]

Closes #225
Closes #224
Closes #222
Closes #223

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Summary

Implements the yield strategy subsystem for the Nestera savings protocol. This includes a generic strategy interface for external contracts, a governance-gated strategy registry, and deposit routing logic for LockSave and GroupSave plans.

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Changes

interface.rs

• Defines the YieldStrategy trait with four entry points: strategy_deposit, strategy_withdraw, strategy_harvest, and strategy_balance
• Uses #[contractclient] to generate YieldStrategyClient for Soroban cross-contract invocation
• Documents expected behavior and security assumptions for implementing contracts

registry.rs

• Introduces StrategyInfo struct (address, enabled, risk_level)
• Implements register_strategy, disable_strategy, get_strategy, get_all_strategies
• All write operations are restricted to admin or governance via validate_admin_or_governance
• Strategies stored in persistent storage with TTL extension on every write

routing.rs

• Implements route_to_strategy following the Checks-Effects-Interactions (CEI) pattern: state is written before the external call, ensuring atomicity if the strategy call reverts
• Tracks principal_deposited and strategy_shares per plan in a StrategyPosition
• Supports both Lock and Group position key variants
• withdraw_from_strategy clears position state before the external call (reentrancy safe)
• Emits StrategyDeposit and StrategyWithdraw events

errors.rs

• Adds three new error variants: StrategyNotFound (92), StrategyAlreadyRegistered (93), StrategyDisabled (94)

lib.rs

• Wires the strategy module and exposes contract-level entry points: register_strategy, disable_strategy, get_strategy, get_all_strategies, route_lock_to_strategy, route_group_to_strategy, get_lock_strategy_position, get_group_strategy_position, withdraw_lock_strategy, withdraw_group_strategy

tests.rs

• 11 new tests covering: strategy registration, duplicate prevention, disable, unauthorized access, routing validation (invalid amount, unregistered strategy, disabled strategy), and empty position reads

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Acceptance Criteria Met

• Only registered and enabled strategies accept deposits
• Strategies stored in persistent storage with proper TTL management
• No reentrancy risk: state is always updated before external calls; Soroban atomicity guarantees rollback on failure
• All 238 tests pass (11 new + 227 existing)
• Contract builds successfully

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