feat(accelerator): numba granularity backend

noxfile.py

@@ -14,7 +14,7 @@ def tests(session: nox.Session) -> None:
             "pytest-cov",
             "pytest-markdown-docs",
             "six",  # centrosome runtime dep, not declared in its metadata
-            ".",
+            ".[numba]",  # exercise the numba backend + correctness harness in CI
         )
     except Exception:
         session.skip(

pyproject.toml

@@ -17,6 +17,9 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+numba = [
+    "numba>=0.59",
+]
 test = [
     "pytest>=8.4.2",
     "pytest-cov",

src/cp_measure/_detect.py

@@ -0,0 +1,14 @@
+"""Backend capability detection.
+
+Detected ONCE at import via ``importlib.util.find_spec`` — availability is
+checked without importing the package or catching ImportErrors. Dispatch reads
+the flag; the resolved backend path is then called directly and unguarded
+(a backend that is flagged present but raises is a real bug and must surface,
+not be papered over by a try/except fallback).
+
+Other backends (jax, ...) add their own flags here as they are wired.
+"""
+
+import importlib.util
+
+HAS_NUMBA: bool = importlib.util.find_spec("numba") is not None

src/cp_measure/bulk.py

@@ -45,6 +45,30 @@
 _3D_FEATURES = ("intensity", "sizeshape", "texture", "granularity")
 
 
+def _numba_registries() -> dict[str, dict[str, Callable]]:
+    """Registries for the 'numba' accelerator.
+
+    Composes the numba implementations (``intensity``, ``granularity``) with the
+    numpy implementations of every other feature — a single global "numba"
+    selection still yields a full, working feature set, accelerated where a
+    numba backend exists. This is explicit per-function composition, NOT an
+    error-driven fallback.
+    """
+    from cp_measure.core.numba import (
+        get_granularity as _numba_granularity,
+        get_intensity as _numba_intensity,
+    )
+
+    return {
+        "core": {
+            **_CORE,
+            "intensity": _numba_intensity,
+            "granularity": _numba_granularity,
+        },
+        "correlation": _CORRELATION,
+    }
+
+
 def _dispatch(name: str) -> dict[str, Callable]:
     from cp_measure import _ACCELERATOR
 
@@ -55,9 +79,14 @@ def _dispatch(name: str) -> dict[str, Callable]:
             f"'jax' accelerator not yet wired for {name} measurements"
         )
     if _ACCELERATOR == "numba":
-        raise NotImplementedError(
-            f"'numba' accelerator not yet wired for {name} measurements"
-        )
+        from cp_measure._detect import HAS_NUMBA
+
+        if not HAS_NUMBA:
+            raise RuntimeError(
+                "accelerator 'numba' selected but numba is not installed; "
+                "you can install it via `pip install cp_measure[numba]`"
+            )
+        return _numba_registries()[name]
     if _ACCELERATOR == "fastest":
         raise NotImplementedError("'fastest' logic not yet implemented")
     raise ValueError(

src/cp_measure/core/numba/__init__.py

@@ -0,0 +1,15 @@
+"""Numba-accelerated backend.
+
+Selected explicitly by import (``from cp_measure.core.numba import get_intensity``)
+or globally via ``cp_measure.set_accelerator("numba")``. Requires the optional
+``numba`` extra; availability is gated by ``cp_measure._detect.HAS_NUMBA``.
+
+This backend accelerates ``intensity`` and ``granularity``; the global "numba"
+accelerator composes them with the numpy implementations of every other feature
+(see ``cp_measure.bulk``).
+"""
+
+from cp_measure.core.numba.measuregranularity import get_granularity
+from cp_measure.core.numba.measureobjectintensity import get_intensity
+
+__all__ = ["get_granularity", "get_intensity"]