feat: Add lightweight aggregation and sketch metrics

[rahulbsw]

Summary

• add destination-level tumbling-window aggregations with count, sum, avg, approx_distinct, and quantiles
• wire aggregation runtime flush lifecycle, processing-time semantics, and validation guards
• add observability, docs, and runnable aggregation examples

Test Plan

• cargo test --quiet
• cargo run --quiet --bin streamforge-validate -- examples/aggregation/orders-windowed-metrics.yaml
• cargo run --quiet --bin streamforge-validate -- examples/aggregation/orders-quantiles.yaml