Emit event when co-signer approves release

[Kingsman-99]

Description

Multi-signature release approval (via sign_release) does not emit an event, so off-chain monitors cannot track how many co-signers have approved without querying storage on every block. An event per approval would enable threshold-based alerting.

Acceptance Criteria

• New cosigner_approved(env, invoice_id, signer, approvals_so_far) event added to events.rs
• Event is emitted from the sign_release / approve_release call path in lib.rs
• approvals_so_far reflects the count after adding the current approval
• Unit test asserts event data matches expected signer and count
• cargo test passes

Context

• Target file: contracts/split/src/events.rs

[The-moving-man]

@The-moving-man has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

please can i take this on

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[grantfox-oss]

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@The-moving-man's PR #712 was approved and merged by @Kingsman-99.

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This issue has been completed by @The-moving-man as part of the Stellar Wave Program's 8th Wave 🥳

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