chore(mypy): add docs/_ext to mypy_path + exclude docs/ for CI parity

tests/docs imports the widget framework as ``from widgets ...`` (the same
way docs/conf.py loads it, via a sys.path insertion). Under mypy strict
that raises "Cannot find implementation or library stub for module named
'widgets'". Listing ``docs/_ext`` under ``mypy_path`` alone -- rather
than adding it to ``files`` -- avoids the reciprocal "source file found
twice under different module names" error that would fire if mypy
discovered the package via both its parent path and as a top-level
module.

``exclude = ["^docs/"]`` is also required so that the CI invocation
``uv run mypy .`` -- which traverses every path under the current
directory -- doesn't rediscover files in ``docs/_ext/widgets/`` under
their ``docs._ext.`` prefix and error with the same "found twice"
message. The widgets package is still fully type-checked via the
``mypy_path`` entry; the exclude only affects discovery-by-traversal.

Both settings are needed from the moment the widgets package was
introduced (17a406a); the ``mypy .`` failure was only surfaced by CI
after tests/docs landed.

Author: tony

pyproject.toml

@@ -115,6 +115,14 @@ files = [
   "src",
   "tests",
 ]
+# ``docs/_ext`` is not in ``files`` to avoid double-naming the widgets package
+# (mypy would see it as both ``widgets`` and ``docs._ext.widgets``); the path
+# entry alone lets imports like ``from widgets import ...`` in tests resolve.
+# ``exclude`` is also required so that ``uv run mypy .`` in CI -- which
+# traverses every path under the current dir -- doesn't rediscover the same
+# files under their ``docs._ext.`` prefix and error "Source file found twice".
+mypy_path = ["docs/_ext"]
+exclude = ["^docs/"]
 
 [[tool.mypy.overrides]]
 module = ["docutils", "docutils.*"]