feat: wire splitter & dispute modules, harden validation, add real tests

[nottherealalanturing]

Summary

Resolves #90, #91, #92, #93 in a single branch.

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Changes

lib.rs — Wire modules (closes #92)

• Declare pub mod dispute and pub mod splitter so both modules compile into the crate.
• Add #[cfg(test)] mod dispute_test and #[cfg(test)] mod splitter_test.

contract.rs — Expose public entrypoints (closes #92)

• Add escrow entrypoints: create_escrow, release_escrow, refund_escrow, get_escrow.
• Add dispute entrypoints: open_dispute, resolve_dispute, get_dispute.
• Add splitter entrypoints: create_split, distribute, get_split.

dispute.rs — Fix resolve_dispute (closes #93)

• Remove broken release_escrow(e, escrow_id) / refund_escrow(e, escrow_id) calls (wrong signatures, required depositor/beneficiary auth).
• Add private settle_escrow_by_outcome helper that directly mutates the escrow record and moves balances — no depositor/beneficiary auth required, only the designated resolver's auth.
• resolve_dispute now compiles cleanly against the real escrow API.

splitter.rs — Harden validation (closes #91)

• Reject empty recipient lists.
• Reject zero-share recipients.
• Reject duplicate recipient addresses.
• Reject non-positive total amounts (already via require_positive_amount, now also guarded before BPS loop).

splitter_test.rs — Real Soroban tests (closes #90)

• Replaced placeholder file (referenced nonexistent Contract, Recipient, calculate_distribution) with real tests using Env, registered contracts, and actual splitter.rs types.
• Covers: create, distribute (2-recipient, 3-recipient rounding), unauthorized distribute, double-distribute, empty recipients, zero-share, duplicate address, non-positive amount.

dispute_test.rs — New test file (closes #93)

• Covers: open dispute, resolve for beneficiary, resolve for depositor, wrong resolver panics, double-resolve panics, open dispute on settled escrow panics.

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Closes #90
Closes #91
Closes #92
Closes #93

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