refactor(eval): split event.rs into module directory

[mostafa]

Summary

• Split crates/rsigma-eval/src/event.rs (758 lines) into event/ module directory
• Extract EventValue enum, Event trait, and blanket impls into event/mod.rs
• Move each concrete implementation to its own file:
  • event/json.rs - JsonEvent + traverse/collect helpers (14 tests)
  • event/kv.rs - KvEvent (3 tests)
  • event/plain.rs - PlainEvent (3 tests)
  • event/map.rs - MapEvent (2 tests)
• EventValue coercion tests remain in event/mod.rs (4 tests)

Test plan

• cargo fmt --all -- --check passes
• cargo clippy --workspace --all-targets --all-features -- -D warnings passes
• cargo test --workspace passes (970+ tests across all crates)