Validate verifier and destination addresses are distinct in create_vault

[1nonlypiece]

📌 Description

create_vault in src/lib.rs accepts verifier, success_destination, and
failure_destination with no relationship checks. The repo already ships
snapshots probing dangerous overlaps (e.g.
test_create_vault_verifier_equals_success_destination_rejected,
test_create_vault_verifier_equals_failure_destination_rejected), but the
constraint is not consistently enforced in the contract itself.

This issue makes the address-distinctness invariant explicit and enforced at
creation time.

Goal: reject vault creation when verifier == success_destination or
verifier == failure_destination, since a verifier who is also a payout target
has a conflict of interest.

🎯 Requirements and Context

• Must be secure, tested, and documented.
• Add a new Error::ConflictingAddresses returned by create_vault.
• Decide and document the rule for success_destination == failure_destination
  (the existing snapshot ..._no_verifier_destination_overlap_allowed suggests
  some overlaps are allowed — keep behaviour consistent with it).
• Preserve verifier == creator acceptance (TEST_VERIFIER_SAME_AS_CREATOR.md).

🛠️ Suggested Execution

1. Fork the repo and create a branch

git checkout -b feature/verifier-address-distinctness

2. Implement changes

• Contract: add the distinctness checks near the top of create_vault in
  src/lib.rs, before pulling USDC into escrow.
• Tests: extend tests/create_vault.rs to assert each rejected/accepted overlap,
  matching the existing snapshot expectations.
• Docs: add docs/ADDRESS_CONSTRAINTS.md enumerating allowed/rejected overlaps;
  cross-link from README.md.
• Add NatSpec-style /// comments documenting the new validation rules.
• Validate security: a verifier can never also be a payout destination.

3. Test and commit

• Run cargo test.
• Cover edge cases: verifier == success, verifier == failure, verifier == creator
  (allow), None verifier with overlapping destinations (per existing snapshot).

Example commit message

feat: enforce verifier/destination address distinctness in create_vault

✅ Guidelines

• Minimum 95% test coverage on new/changed lines.
• Clear documentation of the allowed/rejected address matrix.
• No regressions on existing overlap snapshots.
• Timeframe: 96 hours.

🏷️ Labels

type-enhancement · area-contracts · type-security · type-testing ·
MAYBE REWARDED · GRANTFOX OSS · OFFICIAL CAMPAIGN

💬 Community & Support

• Join the Disciplr contributor Discord to coordinate, ask questions, and get
  unblocked fast: https://discord.gg/xvNAvMJf
• Please introduce yourself before you start so we can avoid duplicate work,
  pair you with a reviewer, and merge your PR quickly.

[dangelo352]

I can take this. I’ll add Error::ConflictingAddresses, enforce verifier != success_destination and verifier != failure_destination before escrow transfer, preserve verifier == creator and no-verifier destination-overlap behavior, then document the address matrix.

[leocagli]

I'd like to work on this issue, please assign me so I can begin implementation.