chore: initialize project scaffold

Add .gitignore (excludes .claude/, venv, checkpoints, processed data,
external corpora), .gitattributes (LF normalization, binary markers),
full directory tree with .gitkeep placeholders, and src __init__ stubs.

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

.gitignore

@@ -0,0 +1,61 @@
+# Python
+__pycache__/
+*.py[cod]
+*.pyo
+*.pyd
+.Python
+*.egg-info/
+dist/
+build/
+*.egg
+.eggs/
+
+# Virtual environments
+.venv/
+venv/
+env/
+ENV/
+
+# pytest
+.pytest_cache/
+.cache/
+htmlcov/
+.coverage
+coverage.xml
+
+# Jupyter
+.ipynb_checkpoints/
+*.ipynb_checkpoints
+
+# Model checkpoints (large binaries — commit only intentionally)
+checkpoints/*.pt
+checkpoints/*.pth
+checkpoints/*.ckpt
+
+# Processed data (reproducible from source)
+data/processed/*.pt
+data/processed/*.pkl
+
+# External corpora (download separately; too large for git)
+data/raw_external/
+
+# OS
+.DS_Store
+Thumbs.db
+desktop.ini
+
+# IDEs
+.idea/
+*.swp
+*.swo
+
+# Claude Code
+.claude/
+
+# Logs
+*.log
+logs/
+
+# Misc
+*.tmp
+*.bak