fix(ci): pin mutmut 3.5+, exclude module-level init from mutation

mutmut 3.x trampoline crashes when called during module import
(parsers/__init__.py creates FragmentationEngine at module level).
Exclude it from mutation and remove redundant pip install mutmut.

Author: nikolay-e

.github/workflows/ci.yml

@@ -160,8 +160,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -e .[dev]
-          pip install mutmut
+          pip install -e ".[dev]"
 
       - name: Run mutation testing
         run: |

pyproject.toml

@@ -147,7 +147,7 @@ dev = [
     "pytest-cov>=3.0,<8.0",
     "pytest-xdist>=3.0,<4.0",
     "hypothesis>=6.0,<7.0",
-    "mutmut>=2.0,<4.0",
+    "mutmut>=3.5,<4.0",
     "coverage>=7.0,<8.0",
     # Quality checks
     "ruff>=0.4,<1.0",
@@ -168,6 +168,14 @@ dev = [
     "urllib3>=2.6.0",
 ]
 
+[tool.mutmut]
+paths_to_mutate = ["src/treemapper/"]
+tests_dir = ["tests/"]
+do_not_mutate = [
+    "src/treemapper/diffctx/parsers/__init__.py",
+    "src/treemapper/diffctx/stopwords.py",
+]
+
 [tool.setuptools.dynamic]
 version = { attr = "treemapper.version.__version__" }