feat(_ext[spf_demo_fixtures]): add synthetic fixtures for badge demo page

why: The badge demo page needs fixtures that exercise every badge
permutation (scope × kind × state) without depending on libtmux's
real fixtures, which only cover a subset of combinations.
what:
- Add spf_demo_fixtures.py with 9 synthetic @pytest.fixture functions
  covering: plain, session/module/class scope, factory kind,
  override_hook kind, autouse, deprecated, and two combinations
  (session+factory, session+autouse)

Author: tony

docs/_ext/spf_demo_fixtures.py

@@ -0,0 +1,79 @@
+"""Synthetic fixtures for the sphinx_pytest_fixtures badge demo page.
+
+Each fixture exercises one badge-slot combination so the demo page can show
+every permutation side-by-side:
+
+  Scope:  session | module | class | function (suppressed — no badge)
+  Kind:   resource (suppressed) | factory | override_hook
+  State:  autouse | deprecated (set via RST :deprecated: option)
+  Combos: session+factory, session+autouse
+
+These fixtures are purely for documentation; they are never collected by
+pytest during a real test run (the module is not in the test tree).
+"""
+
+from __future__ import annotations
+
+import pytest
+
+
+@pytest.fixture
+def demo_plain() -> str:
+    """Plain function-scope resource. Shows FIXTURE badge only."""
+    return "plain"
+
+
+@pytest.fixture(scope="session")
+def demo_session() -> str:
+    """Session-scoped resource. Shows SESSION + FIXTURE badges."""
+    return "session"
+
+
+@pytest.fixture(scope="module")
+def demo_module() -> str:
+    """Module-scoped resource. Shows MODULE + FIXTURE badges."""
+    return "module"
+
+
+@pytest.fixture(scope="class")
+def demo_class() -> str:
+    """Class-scoped resource. Shows CLASS + FIXTURE badges."""
+    return "class"
+
+
+@pytest.fixture
+def demo_factory() -> type[str]:
+    """Return a callable (factory kind). Shows FACTORY + FIXTURE badges."""
+    return str
+
+
+@pytest.fixture
+def demo_override_hook() -> str:
+    """Override hook — customise in conftest.py. Shows OVERRIDE + FIXTURE badges."""
+    return "override"
+
+
+@pytest.fixture(autouse=True)
+def demo_autouse() -> None:
+    """Autouse fixture. Shows AUTO + FIXTURE badges."""
+
+
+@pytest.fixture
+def demo_deprecated() -> str:
+    """Return a value (deprecated since 1.0, replaced by :func:`demo_plain`).
+
+    This fixture is documented with the ``deprecated`` RST option so the
+    demo page can show the DEPRECATED + FIXTURE badge combination.
+    """
+    return "deprecated"
+
+
+@pytest.fixture(scope="session")
+def demo_session_factory() -> type[str]:
+    """Session-scoped factory. Shows SESSION + FACTORY + FIXTURE badges."""
+    return str
+
+
+@pytest.fixture(scope="session", autouse=True)
+def demo_session_autouse() -> None:
+    """Session-scoped autouse. Shows SESSION + AUTO + FIXTURE badges."""