docs(http): pre-decomposition sanity check fixes — /subscribe POST, direct-call cleanup, from_mcp output handling

Three issues found in the http crate spec sanity check that would have
caused problems during task decomposition, now fixed:

C1 — /subscribe GET→POST: the gateway's /subscribe is an invoke endpoint
carrying { operation, input } in the body, but was listed as GET (which
has no body). Flipped to POST with Accept: text/event-stream negotiating
the SSE response, consistent with /call's flat-JSON-body invariant.
Browsers using EventSource can't POST but use WebSocket for the
bidirectional path; the HTTP gateway's /subscribe is for non-browser
HTTP clients (fetch + ReadableStream). Touches ADR-042, ADR-047,
ADR-048, http-adapters.md, http-server.md.

C2 — stale direct-call references: three spots contradicted ADR-047
(which removed the POST /{service}/{op} direct-call surface) and
ADR-046 §3 (which states /{service}/{op} is no longer reserved).
Cleaned up in http-server.md (custom-routes intro + collision list) and
ADR-046 §6 (default-surface list).

W2 — from_mcp output handling: the spec's fallback for tools without
outputSchema was Type.Unknown(), but the correct fallback is the MCP
ContentBlock union (text|image|audio|resource|resource_link) — a
well-defined MCP type, not Unknown. Fixed http-mcp.md with the full
structuredContent-preferred-over-content-blocks logic (matching the TS
adapter and rmcp SDK), enriched references with specific rmcp source
files. Also added shared-dispatch-spine notes to http-mcp.md and
http-adapters.md cross-referencing the new research findings.

Research (docs/research/alknet-http-gateway-factoring/findings.md):
to_mcp and to_openapi share a dispatch spine (resolve → invoke → map).
Recommendation: extract a thin shared struct now, not a GatewayDispatch
trait — the server-integration layers (axum routes vs rmcp
StreamableHttpService) and wire-framing stay per-gateway. A third
gateway is not on the horizon; if one appears its server-integration
needs its own shape anyway.

Minor: WS route precedence note (websocket.md), OpenAPISpec
shared-type-not-shape clarification (http-adapters.md), date bumps.

docs/architecture/crates/http/http-adapters.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-06-30
+last_updated: 2026-07-01
 ---
 
 # HTTP Adapters — from_openapi and to_openapi
@@ -50,7 +50,11 @@ impl OperationAdapter for FromOpenAPI {
 /// implementation detail (openapiv3::OpenApi, a local alknet-http
 /// type, or a serde_json::Value-based parse); the one-way constraint is
 /// that `from_openapi` accepts a standard OpenAPI 3.x JSON/YAML doc and
-/// `to_openapi` produces one. Both directions share the same type.
+/// `to_openapi` produces one. Both directions share the same Rust type,
+/// but not the same document shape: `from_openapi` consumes traditional
+/// per-operation-paths docs (one path per operation), while `to_openapi`
+/// produces the 5-endpoint gateway doc (ADR-042). The type is shared;
+/// the shape is not.
 pub struct OpenAPISpec {
     pub info: OpenAPIInfo,
     pub paths: BTreeMap<String, PathItem>,
@@ -261,7 +265,7 @@ surface problem).
 | `/schema` | `services/schema` | `GET` | Get an operation's full `OperationSpec`. |
 | `/call` | `call.requested` (Query/Mutation) | `POST` | Invoke an operation. Flat JSON body `{ operation, input }`. |
 | `/batch` | multiple `call.requested` | `POST` | Invoke multiple operations. Array of `{ operation, input }`. |
-| `/subscribe` | `call.requested` (Subscription) | `GET` (SSE) | Invoke a streaming operation. `text/event-stream`. |
+| `/subscribe` | `call.requested` (Subscription) | `POST` (SSE) | Invoke a streaming operation. Body `{ operation, input }` (same shape as `/call`); response is `text/event-stream`. |
 
 The input is always a flat JSON body — no path/query/body split to
 reverse-engineer. JSON Schema for the input/output is already in the
@@ -300,6 +304,17 @@ HTTP-specific metadata (which fields are path params, etc.). The
 gateway pattern is the default `to_openapi` projection; the traditional
 projection is additive, not a replacement. See ADR-042 §5.
 
+#### Shared dispatch spine with `to_mcp`
+
+`to_openapi`'s `/call` endpoint and `to_mcp`'s `call` tool share the
+same dispatch spine (resolve identity → build `OperationContext` →
+`OperationRegistry::invoke()` → map `ResponseEnvelope`). The
+wire-framing, discovery, streaming, and server-integration layers are
+per-gateway. See [http-mcp.md](http-mcp.md) §"Shared dispatch spine
+with `to_openapi`" and
+`docs/research/alknet-http-gateway-factoring/findings.md` for the
+factoring recommendation (thin shared struct, not a trait).
+
 ### Error Fidelity (ADR-023)
 
 `from_openapi` maps OpenAPI non-2xx response status codes to