feat: add deposit credit rollover for excess deposits

[Tranquil-Flow]

Summary

When a member deposits more than their fixedDeposit dues, the excess rolls over as credit for future epochs instead of reverting with ExceedsDepositAmount.

Changes

• New mapping: memberDepositCredit[id][member] — tracks excess deposit credit
• New view: getMemberDepositCredit(id, member)
• Modified _deposit():
  • Auto-applies existing credit toward current epoch dues
  • Stores excess above fixedDeposit as credit (no more revert)
  • memberWithdrawableBalance only increases for effective deposits + consumed credit
• Modified decommission(): Returns stored credit to members alongside withdrawable balance
• Updated fuzz tests: SafetyNetFuzz_DepositWithdraw adapted for new credit behavior
• Updated unit tests: ExceedsDepositAmount expectations replaced with credit assertions

Testing

5 new tests: credit accumulation, auto-consumption, partial consumption, excess handling, decommission return. All 102 tests pass.

Closes #54