Introduce an OpenAPI-first HTTP client driven by the same specification

used for server-side route binding, and refactor documentation and
templates to treat the client as a first-class contract consumer.

Key changes:
- Add OpenAPI-first client module based on httpx
- Document client usage alongside server-side binder usage
- Update mkdocs navigation to include client documentation
- Refactor CRUD and model app templates to call APIs via operationId
  instead of hardcoded paths
- Align package documentation and public API surface with client support
- Clarify server/client dependency split (fastapi vs httpx)

This establishes strict symmetry between OpenAPI-driven server binding
and OpenAPI-driven client invocation, reinforcing OpenAPI as the single
source of truth on both sides of the contract.

openapi_first/__init__.py

@@ -17,11 +17,12 @@ convenience facades.
 Architecture Overview
 ----------------------------------------------------------------------
 
-The library is structured around three core responsibilities:
+The library is structured around four core responsibilities:
 
 - loader: load and validate OpenAPI 3.x specifications (JSON/YAML)
 - binder: bind OpenAPI operations to FastAPI routes via operationId
 - app: OpenAPI-first FastAPI application bootstrap
+- client: OpenAPI-first HTTP client driven by the same specification
 - errors: explicit error hierarchy for contract violations
 
 The package root acts as a **namespace**, not a facade. Consumers are
@@ -40,7 +41,8 @@ Or with Poetry:
     poetry add openapi-first
 
 Runtime dependencies are intentionally minimal:
-- fastapi
+- fastapi (server-side)
+- httpx (client-side)
 - openapi-spec-validator
 - pyyaml (optional, for YAML specs)
 
@@ -48,7 +50,7 @@ The ASGI server (e.g., uvicorn) is an application-level dependency and is
 not bundled with this library.
 
 ----------------------------------------------------------------------
-Basic Usage
+Server-Side Usage (OpenAPI → FastAPI)
 ----------------------------------------------------------------------
 
 Minimal OpenAPI-first FastAPI application:
@@ -57,7 +59,7 @@ Minimal OpenAPI-first FastAPI application:
     import my_service.routes as routes
 
     api = app.OpenAPIFirstApp(
-        openapi_path="openapi.json",
+        openapi_path="openapi.yaml",
         routes_module=routes,
         title="My Service",
         version="1.0.0",
@@ -81,6 +83,56 @@ OpenAPI snippet:
             "200":
               description: OK
 
+The binder guarantees:
+- Every OpenAPI operationId has exactly one handler
+- No undocumented routes exist
+- All mismatches fail at application startup
+
+----------------------------------------------------------------------
+Client-Side Usage (OpenAPI → HTTP Client)
+----------------------------------------------------------------------
+
+The same OpenAPI specification can be used to construct a strict,
+operationId-driven HTTP client.
+
+Client construction:
+
+    from openapi_first.loader import load_openapi
+    from openapi_first.client import OpenAPIClient
+
+    spec = load_openapi("openapi.yaml")
+
+    client = OpenAPIClient(spec)
+
+Calling operations (operationId is the API):
+
+    response = client.get_health()
+    assert response.status_code == 200
+    assert response.json() == {"status": "ok"}
+
+Path parameters must match the OpenAPI specification exactly:
+
+    response = client.get_item(
+        path_params={"item_id": 1}
+    )
+
+Request bodies are passed explicitly:
+
+    response = client.create_item(
+        body={"name": "Orange", "price": 0.8}
+    )
+
+Client guarantees:
+- One callable per OpenAPI operationId
+- No hardcoded URLs or HTTP methods in user code
+- Path and body parameters must match the spec exactly
+- Invalid or incomplete OpenAPI specs fail at client construction time
+- No schema inference or mutation is performed
+
+The client is transport-level only and returns `httpx.Response`
+objects directly. Response interpretation and validation are left to
+the consumer or higher-level layers.
+
 ----------------------------------------------------------------------
 Extensibility Model
 ----------------------------------------------------------------------
@@ -107,6 +159,7 @@ The supported public API consists of the following top-level modules:
 - openapi_first.app
 - openapi_first.binder
 - openapi_first.loader
+- openapi_first.client
 - openapi_first.errors
 
 Classes and functions should be imported explicitly from these modules.
@@ -118,8 +171,8 @@ Design Guarantees
 
 - OpenAPI is the single source of truth
 - No undocumented routes can exist
-- No OpenAPI operation can exist without a handler
-- All contract violations fail at application startup
+- No OpenAPI operation can exist without a handler or client callable
+- All contract violations fail at application startup or client creation
 - No hidden FastAPI magic or implicit behavior
 - Deterministic, testable application assembly
 - CI-friendly failure modes
@@ -131,11 +184,13 @@ enforcement over convenience shortcuts.
 from . import app
 from . import binder
 from . import loader
+from . import client
 from . import errors
 
 __all__ = [
     "app",
     "binder",
     "loader",
+    "client",
     "errors",
 ]