TODO

Allocations

&'static str values stored as String in headers (response.rs:10) — ct.to_string() and "gzip" are heap-allocated despite being static. Change the header map value type to Cow<'static, str> so static values avoid allocation.
as_bytes() builds an intermediate Vec<u8> (response.rs:32) — every response allocates a full buffer before writing to the stream. Replace with write_to(&self, w: &mut impl Write) to write headers and body directly.

MIME type values mis-grouped in headers module (http.rs:12-13) — TEXT_PLAIN and OCTET_STREAM are content-type values, not header names; they don't belong alongside CONTENT_TYPE etc.
Non-deterministic header order (response.rs:37) — HashMap iteration is randomized per-process, making response headers non-reproducible across runs. IndexMap or BTreeMap would give stable ordering for easier debugging and testing.