Add kaya-openapi package for automatic OpenAPI spec generation

- New packages/kaya-openapi with OpenAPIMixin, @operation decorator,
  and generate_spec() that walks the routing tree
- Enables kaya-core's Tree.register to expose the original handler
  callback as an instance attribute for metadata introspection
- Registers GET /openapi.json and GET /docs (Swagger UI) routes
- Supports  and  path parameters, docstring
  descriptions, @operation metadata, and excludes wildcard/WS routes
- Adds example/openapi.py, updates CI, README, and requirements

packages/kaya-openapi/README.md

@@ -0,0 +1,93 @@
+# kaya-openapi
+
+Automatic [OpenAPI](https://www.openapis.org/) specification generation for the
+[Kaya](https://github.com/woggioni/kaya) lightweight ASGI web framework.
+
+The package provides an `OpenAPIMixin` that inspects a `KayaApp`'s routing
+tree and serves:
+
+- an OpenAPI 3.1 JSON document (default: `GET /openapi.json`)
+- a Swagger UI page to browse it interactively (default: `GET /docs`)
+
+## Usage
+
+```python
+from kaya.core import HttpContext, KayaApp
+from kaya.openapi import OpenAPIMixin, operation
+
+app = KayaApp(mixins=[OpenAPIMixin(title='My API', version='1.0.0')])
+
+
+@app.GET('/users/${user_id:int}')
+@operation(summary='Get a user',
+           tags=['users'],
+           responses={
+               200: {'description': 'The user'},
+               404: {'description': 'User not found'},
+           })
+async def get_user(ctx: HttpContext, user_id: int) -> None:
+    ...
+```
+
+Run the app with any ASGI/RSGI server and open `http://localhost:8000/docs`.
+
+## How routes are mapped
+
+- Static segments and parameters are converted to OpenAPI path templating:
+  - `/users/${user_id}` → `/users/{user_id}` (string path parameter)
+  - `/users/${user_id:int}` → `/users/{user_id}` (integer path parameter)
+- Wildcard routes (`*`) are **skipped**: they cannot be expressed in OpenAPI
+  path syntax.
+- Websocket routes are **skipped**: OpenAPI does not model websockets.
+- Method-agnostic routes (registered with `app.route(path)` without methods)
+  are documented under **all** standard HTTP methods, since they respond to
+  all of them.
+- The mixin's own endpoints are excluded from the document unless
+  `include_self=True`.
+
+The document is generated on every request to the spec endpoint, so routes
+registered after the mixin is applied are always included.
+
+## Operation metadata
+
+The `@operation` decorator attaches OpenAPI metadata to a route handler.
+All fragments are plain dicts merged verbatim into the generated operation
+object, so any valid OpenAPI 3.1 construct can be used:
+
+```python
+@operation(summary='...',            # operation summary
+           description='...',        # defaults to the handler docstring
+           tags=['users'],
+           operation_id='getUser',
+           request_body={...},       # OpenAPI requestBody object
+           responses={200: {...}},   # per-status-code response objects
+           parameters=[...],         # extra/overriding parameter objects
+           deprecated=False,
+           hidden=False)             # exclude from the document
+```
+
+`parameters` entries whose `name` and `in` match an auto-generated path
+parameter override it; all others are appended.
+
+## Configuration
+
+```python
+OpenAPIMixin(
+    title='My API',                # info.title (required)
+    version='1.0.0',               # info.version (required)
+    description='',                # info.description
+    spec_path='/openapi.json',     # where the JSON document is served
+    docs_path='/docs',             # where Swagger UI is served
+    servers=[{'url': 'https://api.example.com'}],
+    openapi_version='3.1.0',
+    include_self=False,            # include spec/docs endpoints in the document
+)
+```
+
+The document can also be generated programmatically without serving it:
+
+```python
+from kaya.openapi import generate_spec
+
+spec = generate_spec(app, title='My API', version='1.0.0')
+```