docs(http): add ADR-048 and websocket.md — WS carries native session, not gateway

Promote the WebSocket browser path from a section in http-server.md to a
first-class spec (websocket.md) and commit the contract-pattern decision
(ADR-048): a WS connection carries the native EventEnvelope call-protocol
session, not the HTTP gateway shape. The gateway endpoints are HTTP-only;
discovery on WS is via services/list/services/schema as ordinary call-protocol
ops; subscriptions project as native call.responded events (no SSE).

ADR-044 already decided WS as the v1 browser bidirectional path; ADR-048
clarifies the shape of what ADR-044 committed (§1 implies native session;
the ADR makes it an explicit implementer-visible rule). The from_wss adapter
(importing a remote node's ops over WS) is recorded as out-of-scope with a
concrete reversal trigger so it is not re-derived later.

Spec cleanup: http-server.md WS section collapsed to a stub pointer;
websocket.md Why section references ADRs rather than re-arguing them;
length-prefix decision made canonical (no prefix on WS — message boundary
is the delimiter); default upgrade path pinned (/alknet/call) with HTTP/2
extended CONNECT noted; indexes (README, http/README, overview) updated.

docs/architecture/crates/http/webtransport.md

@@ -9,7 +9,7 @@ last_updated: 2026-06-30
 > This spec is kept intact for revival. `h3`/WebTransport is not
 > implemented in the initial `alknet-http` release; the browser
 > bidirectional path uses WebSocket (see
-> [http-server.md](http-server.md) §"WebSocket browser path"). ADR-038
+> [websocket.md](websocket.md)). ADR-038
 > is superseded; ADR-040 and ADR-043 are parked (their decisions revive
 > unchanged when WebTransport revives). The reversal trigger is a
 > concrete deployment needing the ALPN-stream-proxy (a browser running