Add snapshot tests for every public event

[P3az3]

Problem Statement

Smart contract events are the contract's public observability surface. The tests verify state but never directly assert the serialized event shape. A future change to event::donation_received (e.g. accidental field reordering, wrong tuple shape) would not be caught until integration with off-chain tooling.

Expected Outcome

Add a snapshot-style integration test that emits a representative flow (initialize, donate x3, release x1) and serializes the events via env.events().all() (or equivalent). Compare against a snapshot fixture.

Acceptance Criteria

• New test tests/event_snapshots.rs exercising a representative flow.
• Snapshot file tests/snapshots/event_flow.snap serialized as a JSONL or known text format.
• CI gates any change to the event shape.

Implementation Notes

• env.events() returns a Events iterator in Soroban 26.x; serialize each into text for diffing.

Affected Files / Modules

• New: campaign/tests/event_snapshots.rs (uses insta crate)
• campaign/Cargo.toml (dev-dep insta)
• New: campaign/tests/snapshots/event_flow.snap

Dependencies — None.