fix: add explicit InsufficientPoolLiquidity checks

[Tranquil-Flow]

Summary

Adds explicit InsufficientPoolLiquidity error checks before pool balance decrements, replacing opaque Panic(0x11) arithmetic underflow reverts with a named, catchable error.

Changes

• ISafetyNet.sol: New error InsufficientPoolLiquidity()
• SafetyNet.sol: Two guard insertions:
  1. _withdraw() small path — before safetyNetBalance[_id] -= _withdrawAmount
  2. _deduct() large path — before safetyNetBalance[_safetyNetId] -= _amount

Why

Normal accounting keeps memberWithdrawableBalance ≤ safetyNetBalance, so these guards shouldn't fire in practice. But if a bug, admin action, or future upgrade ever de-synchronises the two balances, the contract would previously emit an opaque Panic(0x11). The explicit check makes on-chain debugging and client-side error handling straightforward.

Testing

3 new tests including 2 using vm.store to simulate de-synchronised accounting. All 100 tests pass.

Closes #30