Parent: #134
Child of #140, #163, and #164. Implement all 25 pinned WebCore__FetchHeaders__* entries against one exact Headers representation, including standalone native handles and consumer adapters.
Scope
- Complete the shared create, createFromJS, cast, clone, cloneThis, toJS, ref-lifetime, count, copy, append, put, get, has, remove, isEmpty, and fast-header operations.
- Implement the pinned UWS, H3, PicoHeaders, and UWS response adapter boundaries from their exact revisioned layouts; do not guess or dereference unvalidated foreign records.
- Implement createValue/createValueNotJS from the pinned StringPointer/ZigString span contract.
Acceptance
- JavaScript-created and native-created Headers values share one canonical ordered representation with an unspoofable engine brand.
- Header names and values follow Fetch validation and normalization, duplicate combination, Set-Cookie separation, insertion ordering, and WebIDL exception behavior.
- Handles have explicit VM/realm ownership and native lifetime semantics; null, stale, foreign-VM, and exception paths are deterministic.
- Fast HTTPHeaderName indexes map exactly to the pinned enum and have parity with general get/has/remove operations.
- copyTo and callback/output strings have documented ownership and remain valid for the consumer-required lifetime.
- External request/response adapters are covered by exact-layout fixtures and reject unsupported layouts rather than exposing partial behavior.
- Add focused engine tests plus compiled Home and Bun declaration/runtime fixtures in Debug and ReleaseSafe.
- Refresh both inventories, concise ABI/API docs, CI, and parent progress. Root benchmark numbers change only after a new accepted benchmark run.