chore(soroban): feature distinct-destinations

[licette32]

Summary

Rejects create_vault configurations where success_destination == failure_destination.
Equal destinations make the success/failure outcome financially indistinguishable for the
creator, removing the accountability incentive that is the core purpose of the vault.

Changes

src/lib.rs

• Added SameDestination = 10 variant to Error enum
• Added validation in create_vault: rejects when success_destination == failure_destination
• Updated # Validation Rules Rustdoc on create_vault to document the new rule and its UX rationale

tests/create_vault.rs

• test_same_destination_rejected — verifies Error::SameDestination (code Add custom error types for contract failures #10) is returned
• test_different_destinations_accepted — explicit positive case for the new constraint

vesting.md

• Updated create_vault → Requirements with new validation rule
• Added item 5 to Known Limitations & Security Notes explaining the UX and security rationale

README.md

• Updated Vault Constraints → Validation Rules with the new constraint

Test Results

Coverage

97.41% (113/116 lines) — above the 95% threshold

Security Notes

• Validation occurs before any state mutation or token transfer (CEI-safe)
• No new dependencies introduced
• Does not affect existing vaults or other contract functions

Closes #124

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