fix: contract address guard, re-entrancy protection, dispute-locked refund, and BPS sum validation

[prismn]

Summary

This PR addresses four security and correctness issues across the contract:

Changes

#434 — Prevent tokens being sent to the contract address via transfer and mint

• Added InvalidRecipient guard in transfer (was missing; already present in transfer_from and mint for the minting path)
• Tokens sent directly to the contract address become permanently unrecoverable; users must use create_escrow instead

#431 — Re-entrancy protection in distribute

• Moved record.distributed = true and the storage write to BEFORE the distribution loop
• Any re-entrant call to distribute now sees distributed = true immediately and panics, closing the double-distribution window

#428 — Block refund_escrow when a dispute is active

• Added check in try_refund_escrow for an open EscrowDispute record
• Depositor can no longer bypass dispute resolution by self-refunding; panics with DisputeOpen
• Added test test_refund_escrow_with_open_dispute_panics

#425 — Validate that share_bps values sum to exactly 10000 in create_split

• Validation already present; added the required named tests:
  test_create_split_shares_sum_to_10000_ok, test_create_split_shares_sum_to_9999_panics, test_create_split_shares_sum_to_10001_panics

Also fixed pre-existing compilation errors in the upstream branch (duplicate imports, duplicate enum variants, symbol length violations, missing imports) to restore a clean library build.

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Closes #434
Closes #431
Closes #428
Closes #425

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