Add reactive event handler runtime

[KaiCode2]

Summary

• add a default-enabled reactive module with provider-neutral event inputs, pure handlers/hooks, route matching, canonical ordering, dedupe, reports, and cache effect application
• add pending-event validation, conflict detection before mutation, invalidation lowering, resync/speculative reporting, and an EventDecoderHandler adapter
• document the runtime in README/CHANGELOG and cover the acceptance paths with focused integration tests

Validation

• cargo test --test reactive_runtime
• cargo test --test event_pipeline
• cargo test --test public_release_surface
• cargo test
• cargo test --no-default-features
• cargo fmt --all --check
• cargo clippy --all-targets --no-deps -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --no-default-features
• cargo +1.88.0 check --lib --locked
• git diff --check

Notes

• AlloySubscriber is intentionally a scaffold and returns Unsupported; live transport wiring, executing resync scheduling, and full reorg journaling remain follow-up work.