Drain header-echo requests before replying

[noise64]

• drains request bodies in the referrer and credentials echo fixtures before constructing their responses
• contains the recurring stock-P3 Hyper IncompleteMessage race without changing runtime code, assertions, or dedicated early-response coverage
• passes the complete stock-P3 runtime group 2 (38/38), focused P2 coverage, root build/format/Clippy, the full skeleton Clippy matrix, and TypeScript tooling
• TODO: the upstream Wasmtime P3 final-response polling defect remains separate; this PR is deterministic test-fixture containment, not a transport fix