docs: port architecture specs and ADRs from alknet-http; write new alkhttp ADRs 067-070

Phase 1 (SDD) — architecture documentation:

Ported specs (adapted for alkcall, producer/consumer terms, 6-endpoint
gateway, channels-over-WS, Sub/Pub operation types):
- overview.md, http-server.md, http-adapters.md, http-mcp.md
- README.md index (rewritten for alkhttp)

New ADRs:
- 067: WebSocket carries the channels protocol (8-byte chunk demux,
  channel 0 = alk/call, upgrade path /alk/channels)
- 068: gateway /publish endpoint for Pub operations (NDJSON body)
- 069: WebTransport out of scope in alkhttp (alknet concern)
- 070: from_wss consumer adapter (wss feature, tokio-tungstenite)

Ported ADRs (25, same numbers, port notes + amendments where the
extraction changed facts): 001-004, 010, 014, 015, 017, 022, 023, 027,
034, 036, 037, 039, 041, 042, 044, 045, 046, 047, 048, 049, 051, 066.

websocket.md rewritten for the channels session; open-questions.md
seeded (OQ-01 WS byte-stream adapter, OQ-02 /publish framing,
OQ-03 from_wss reconnect, OQ-04 browser client ownership).

Verified: cargo test, clippy -D warnings, fmt, doc --no-deps.

src/adapters/mod.rs

@@ -0,0 +1 @@
+

src/client/mod.rs

@@ -0,0 +1 @@
+

src/gateway/mod.rs

@@ -0,0 +1 @@
+

src/lib.rs

@@ -6,4 +6,4 @@ pub mod adapters;
 pub mod client;
 pub mod gateway;
 pub mod server;
-pub mod websocket;
+pub mod websocket;

src/server/mod.rs

@@ -0,0 +1 @@
+

src/websocket/mod.rs

@@ -0,0 +1 @@
+