feat(escrow): add raise_dispute and resolve_dispute

[Agbasimere]

Closes: #315

Summary

Implements the escrow dispute lifecycle for arbiter-backed contracts.

What changed

• Added raise_dispute(contract_id, caller) for client/freelancer escalation from Funded and PartiallyFunded
• Added resolve_dispute(contract_id, arbiter, resolution) for arbiter settlement
• Added DisputeResolution with:
  • FullRefund
  • PartialRefund (fixed 70/30 split favoring client)
  • FullPayout
  • Split(client_amount, freelancer_amount)
• Added arbiter-aware contract creation via create_contract_with_arbiter(...)
• Enforced dispute state-machine guards so milestone release is blocked while disputed
• Enforced post-resolution accounting:
  • released_amount + refunded_amount == total_deposited
  • existing check_accounting_invariant still holds
• Updated dispute documentation to match the shipped API and resolution behavior

Security Notes

• raise_dispute requires authenticated caller and restricts access to client/freelancer only
• resolve_dispute requires authenticated caller and exact match with stored contract.arbiter
• Arbiter-less contracts fail closed and cannot enter dispute flow
• Custom Split rejects negative values and rejects totals that do not match the remaining escrow balance
• Resolution uses checked arithmetic and revalidates accounting invariants before persisting
• Paused/emergency states block both dispute entrypoints
• No new persistent dispute metadata storage was introduced, so there is no added TTL surface

Testing

Covered in unit tests:

• client can raise dispute
• freelancer can raise dispute on partially funded contract
• non-party cannot raise dispute
• contract without arbiter cannot raise dispute
• full refund resolution
• partial refund 70/30 resolution on remaining balance
• custom split resolution
• invalid split rejection
• non-arbiter cannot resolve
• non-disputed contract cannot resolve
• release is blocked while disputed
• pause blocks both dispute entrypoints

Validation

• Passed: cargo check -p escrow --tests
• Could not complete on this machine: cargo test -p escrow

Reason:

• Windows GNU toolchain fails while linking the contract cdylib due to export ordinal limits
• Installed MSVC toolchain is missing link.exe

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