fix(contracts): embed nullifiers vector in Circle state to prevent archival replays (#254)

[midexol]

Summary

Resolves #254

Previously, the contract stored nullifiers as standalone persistent storage entries (\DataKey::Nullifier(circle_id, nullifier_hash)). While \Circle\ persistent entries are re-extended on every \und\ or \claim, write-once nullifier entries were only extended once at creation time (\LEDGER_EXTEND_TO = 500_000\ ledgers). Once archived, \env.storage().persistent().has(&nullifier_key)\ would return \alse, opening a security vulnerability where a claim could be replayed after TTL expiry.

Solution

1. Embedded Nullifiers Vector: Added \pub nullifiers: Vec\ to \pub struct Circle. Storing nullifiers inside the \Circle\ persistent entry ensures nullifiers share the circle's continuously-extended TTL lifecycle and cannot be archived independently while the circle is active.
2. ADR 004: Created \docs/adr/004-storage-archival.md\ analyzing the trade-offs of storage archival options and documenting the decision to embed a bounded \Vec\ in \Circle.
3. Tests & Threat Model: Added
   ullifier_fence_survives_ttl_expiry\ test in \contracts/sharibo/src/test.rs\ to verify the fence after ledger advancement, and updated \docs/threat-model.md.