Finish HTTP/2 stream routing cleanup

[kacy]

Summary

• route downstream HTTP/2 streams independently instead of pinning a connection to the first matched service
• split routed HTTP/2 idle timeout from per-request timeout with new http2_idle_timeout_ms route config
• refactor HTTP/2 connection handling into a dedicated router module and update docs/status surfaces

Validation

• YOQ_SKIP_SLOW_TESTS=1 zig build test
• Result: 1736 passed; 41 skipped; 0 failed