feat: add balance verification to _release()

[marvs8]

Summary

Adds a contract balance check to _release() before routing funds to recipients, as described in issue #32.

Changes

• lib.rs: In _release(), call token_client.balance(&env.current_contract_address()) and assert it is >= invoice.funded before the transfer loop. Panics with "insufficient contract balance" if not.
• test.rs: Added test_release_panics_on_low_balance — pays an invoice partially, drains the contract balance via TokenClient::transfer, then pays the remainder triggering auto-release, which panics as expected.

Acceptance Criteria

• Balance check runs before any recipient transfer in _release()
• Panics with "insufficient contract balance" if balance is too low
• Normal release with correct balance is unaffected
• Test mocks a low balance scenario and verifies panic
• All existing cargo test pass (8/8)
• cargo clippy passes with zero warnings

Closes #32

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