chore: project review (dev tools in extras, dep upgrades, optional-deps guard, stale cleanup)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/remove_ai_watermarks/optional_deps.py

@@ -0,0 +1,28 @@
+"""Shared availability guard for optional dependencies.
+
+A bare ``importlib.util.find_spec(name) is not None`` check lies when only a
+leftover data directory exists in site-packages: e.g. ``trustmark`` downloads
+model weights into its own package dir, so after the package is uninstalled
+(``uv sync`` pruning an extra) a ``trustmark/models/`` remnant survives and
+``find_spec`` resolves it to a namespace-package spec (``loader is None``)
+while the actual import fails. Every ``is_available()`` guard routes through
+``module_available`` so a pure namespace package counts as absent.
+"""
+
+from __future__ import annotations
+
+import importlib.util
+
+
+def module_available(*names: str) -> bool:
+    """True when every named module resolves to a real, importable package.
+
+    A spec with ``loader is None`` is a pure namespace package -- for our
+    optional deps that means a stale directory remnant, not an installed
+    package -- so it is treated as not available.
+    """
+    for name in names:
+        spec = importlib.util.find_spec(name)
+        if spec is None or spec.loader is None:
+            return False
+    return True