userState key under which the list of recorded push promises lives on HttpContext. Stored as `PushPromise list` boxed once.

Honour the peer's SETTINGS_ENABLE_PUSH value. RFC 7540 §6.5.2: "Initial value: 1, which indicates that server push is permitted." If the client has never sent SETTINGS, callers should use `defaultSetting` whose `enablePush = true`.

Return the push promises recorded on the context, in the order they were added. Empty if none. Tolerates a null `userState` (e.g. on a synthetic `HttpContext.empty`).

Allocate the next promised stream id from a server-side counter. RFC 7540 §5.1.1: streams initiated by the server use even-numbered identifiers and the value 0x0 is reserved. Mutates the counter; the next call returns the next even id. Returns `None` if the counter would overflow the legal 31-bit space, in which case the caller MUST send a GOAWAY and stop creating streams.

Public WebPart: record a push intent for `path` with the given request header fragment. No-op under HTTP/1.x — the recorded list is consumed only by the HTTP/2 writer. Multiple calls append; identical paths are not deduplicated (a server may legitimately push the same path twice with different headers, and dedup is the writer's policy).

Check whether starting another stream would respect the peer's SETTINGS_MAX_CONCURRENT_STREAMS. `currentActive` is the number of streams the server currently has Open or HalfClosed (per RFC 7540 §5.1.2 the limit counts those states). When the peer has not advertised a limit (`None`), pushing is permitted.