Migrate integration tests to eventcore-examples

[jwilger]

Summary

Migrate integration tests from individual crates to eventcore-examples and update them to use the projection-based testing pattern.

Context

Per ADR-031 and ADR-032:

• Integration tests should verify results via projections (using EventCollector)
• Integration tests belong in eventcore-examples crate
• Unit tests stay inline with source code

Tests to Migrate

From eventcore/tests/:

• I-001-single_stream-command_test.rs
• I-003-retry_policy_test.rs
• I-004-multi_stream_atomic_test.rs
• I-007-dynamic_stream_discovery_test.rs
• projection_runner_test.rs
• projection_event_retry_config_test.rs
• projection_poll_retry_test.rs

From eventcore-macros/tests/:

• I-006-command_derive_macro_test.rs

Migration Steps per Test

1. Move file to eventcore-examples/tests/
2. Update imports to use public APIs only
3. Replace store.read_stream() with EventCollector + run_projection()
4. Verify test still passes

Dependencies

• Implement EventCollector projector in eventcore-testing (ADR-031) #278 - EventCollector must be implemented first
• Create eventcore-examples crate (ADR-032) #279 - eventcore-examples crate must exist first

Parent Issue

Sub-issue of #272