feat: enforce verifier address distinctness

[dangelo352]

Summary

• add Error::ConflictingAddresses without renumbering existing contract errors
• reject create_vault when verifier equals success_destination or failure_destination before escrow transfer
• preserve verifier == creator and no-verifier destination-overlap behavior
• document the address matrix in docs/ADDRESS_CONSTRAINTS.md and link it from README
• update contract-interface.json error metadata

Closes #225

Validation

• cargo fmt --check
• cargo test
• git diff --check

Note: local cargo tarpaulin --config tarpaulin.toml --out Stdout is unavailable because cargo-tarpaulin is not installed in this environment; CI coverage should run the repository coverage workflow.

[1nonlypiece]

hey, good work, but this can't go in until you're assigned to #225 — that's our process to avoid two people on the same thing. closing for now; claim the issue first and re-raise. https://discord.gg/xvNAvMJf