Function run_aggregation 

pub async fn run_aggregation(
    spec: &AggregationSpec,
    resolver: &dyn ServiceResolver,
) -> ComposeResult

The async aggregation runtime: fetches all fragments, shapes each, and composes the result.

This is the runtime half of the aggregation module (DW-061 spec + DP-09 runtime). It uses simple async TCP with manual HTTP/1.1 framing to fetch each fragment from its upstream service, then feeds the results through the pure compose function.

Fragments are fetched sequentially (the resolver is a borrowed reference, so spawning parallel tasks would require 'static). A future optimization can wrap the resolver in Arc and use JoinSet for parallel fetches.