docs: add comprehensive NatSpec for redeemRatio and withdrawal formula

[Tranquil-Flow]

Summary

Adds detailed NatSpec documentation for the redeemRatio mechanism and withdrawal formula — the core economic math of Safety Net.

Changes

• ISafetyNet.sol: Expanded redeemRatio param docs — multiplier semantics, valid range (1-22), formula
• SafetyNet.sol:
  • MINIMUM_REDEEM_RATIO / MAXIMUM_REDEEM_RATIO: Expanded NatSpec explaining the ratio caps
  • _withdraw(): 24-line NatSpec with step-by-step formula and worked example
  • _getDailyWithdrawableAmount(): 18-line NatSpec with numbered formula steps

Testing

1 new test test_RedeemRatioFormulaVerification — creates a Safety Net with redeemRatio=5, verifies:

• Initial deposit → 5x withdrawable balance
• Epoch deposit → 5x additional withdrawable
• 1-day withdrawal = exact computed amount (1,666,666,666,666,666,666 wei)

All 98 tests pass.

Closes #29