feat: add harvest strategy yield accounting

[dinahmaccodes]

Closes #227

PR Description

Add Harvest & Yield Accounting for Yield Strategies

Implements yield tracking and harvesting for external yield strategies in the Nestera protocol.

PR Checklist

• harvest_strategy(strategy_id) implemented
• Profit calculated as strategy_balance - principal
• Yield allocation: portion to users, portion to protocol treasury
• Accounting records updated (StrategyTotalPrincipal, StrategyYield, TotalBalance for treasury)
• Yield calculated correctly (tests pass)
• Treasury allocation correct (tests validate fee math)
• No double-counting (no-profit guard short-circuits; harvest-twice test validates)
• Tests validate accounting (16 new tests, 179 total passing)

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