docs: clarify decommission and missed-deposit behavior in NatSpec

[Tranquil-Flow]

Summary

Closes #96

Adds comprehensive NatSpec documentation to clarify the existing decommission and missed-deposit behavior. As discussed in #96, the current behavior is correct for v1 — this PR documents it so the contract's rules are unambiguous to developers, auditors, and frontend integrators.

What's documented

ISavingCircles.sol:

• CircleState enum: detailed description of each state (NotStarted, Active, DepositInProgress, DepositComplete, Expired, Decommissioned, MissedDeposit) and when transitions occur
• CircleDecommissioned event: clarifies it fires after all recoverable funds have been returned
• CircleTimedOut error: explains the exact condition (round window elapsed + incomplete deposits)
• deposit(), depositFor(): documents partial deposit support, CircleTimedOut revert conditions, and that decommission must be called first to recover from missed-deposit state
• withdraw(), withdrawFor(): documents the 5 conditions for withdrawal eligibility, payout amount formula, auto-deactivation on last claim
• decommission(): full flow documentation including preconditions, refund mechanics, storage deletion, and post-decommission state

SavingCircles.sol:

• decommission(): inline documentation of the decommissionable conditions and refund flow
• isWithdrawable(): documents O(N) iteration and return semantics
• _isDecommissionable(): documents the time + state conditions and fund recovery purpose

No behavioral changes. Documentation only.

Test plan

• forge test passes (140 tests)
• No code changes — NatSpec only