style: apply Black formatting to all Python files

Add [tool.black] config to pyproject.toml and Black hook to
.pre-commit-config.yaml (rev 26.3.1), then auto-format all Python
source and test files with line-length=100, target-version=py310.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -9,3 +9,10 @@ repos:
       - id: check-added-large-files
         args: ['--maxkb=500']
       - id: check-merge-conflict
+
+  - repo: https://github.com/psf/black
+    rev: 26.3.1
+    hooks:
+      - id: black
+        language_version: python3
+        args: [--config=pyproject.toml]