docs(http): add ADR-042 OpenAPI gateway pattern for to_openapi

The to_openapi spec was describing one OpenAPI path per alknet operation
— the inverse of from_openapi. That inverse is genuinely messy: the call
protocol's input is a flat JSON object, and generating a traditional
OpenAPI path entry (POST /fs/{path} with path param, body, query params)
requires reverse-engineering which fields are path/query/body — metadata
the call protocol doesn't carry. The three options (leaky HTTP metadata
on OperationSpec, fragile heuristics, manual annotation) are all messy.

ADR-042 replaces this with the gateway pattern (same as ADR-041 for
to_mcp): to_openapi generates 5 fixed endpoints (search, schema, call,
batch, subscribe) that gate access to the full operation registry. The
input is always a flat JSON body — no path/query/body split to
reverse-engineer. JSON Schema is already in the OperationSpec.

The per-caller API surface is the key advantage: /search is
AccessControl-filtered, so the client sees only what it can call. The
Gitea failure mode (dumping admin ops to every caller in a static
OpenAPI doc) is structurally impossible — the per-caller surface is the
default, not an afterthought. OpenAPI has no per-caller filtering
concept; the gateway pattern provides it through /search.

Gateway endpoint set:
- /search -> services/list (AccessControl-filtered, names + descriptions)
- /schema -> services/schema (full OperationSpec)
- /call -> call.requested (Query/Mutation, flat JSON body)
- /batch -> multiple call.requested (correlated IDs)
- /subscribe -> call.requested (Subscription, SSE) — the one endpoint
  the MCP gateway excludes (MCP is request/response; OpenAPI/SSE
  supports streaming)

A traditional per-operation-paths projection is additive (a deployment
that wants the nice Swagger UI builds it with HTTP-specific metadata),
not a replacement. The gateway is the default.

http-adapters.md to_openapi section rewritten: the gateway endpoint
set, per-caller filtering, error fidelity on the /call endpoint, and
the additive traditional projection. The 'Why' section adds the
flat->structured and per-caller-surface rationale.

README/overview ADR tables and the top-level README current-state note
updated for ADR-042.

docs/architecture/crates/http/overview.md

@@ -207,6 +207,7 @@ verified against this invariant. See ADR-014 and
 | HTTP server + client host colocated | [ADR-039](../../decisions/039-http-server-and-client-host-colocated.md) | One crate for server + adapters (shared HTTP deps, shared mapping) |
 | WebTransport ALPN-stream-proxy | [ADR-040](../../decisions/040-webtransport-alpn-stream-proxy.md) | Browser → WebTransport stream → any ALPN handler (SSH, git, SFTP) via WASM parser |
 | `to_mcp` tool-gateway pattern | [ADR-041](../../decisions/041-mcp-tool-gateway-pattern.md) | 4 fixed gateway tools (search/schema/call/batch), not one tool per operation |
+| `to_openapi` gateway pattern | [ADR-042](../../decisions/042-openapi-gateway-pattern.md) | 5 fixed gateway endpoints (search/schema/call/batch/subscribe); per-caller AccessControl-filtered |
 | `alknet-call` is protocol-foundation | [ADR-003](../../decisions/003-crate-decomposition.md) Am. 1 | `alknet-http` depends on `alknet-call` (types, not peer handler) |
 | Bearer auth via `resolve_from_token` | [ADR-004](../../decisions/004-auth-as-shared-core.md) | HTTP handler credential source + resolution (settled) |
 | Stealth mode = HTTP handler on standard ALPNs | [ADR-010](../../decisions/010-alpn-router-and-endpoint.md) | Decoy for unknown paths (settled) |