Modernize s3tree as typed library

[sanketsaurav]

Summary

• Modernize S3Tree as a typed library-first package with pyproject.toml, Python 3.10+ support, py.typed, Ruff, ty, pytest coverage, and GitHub Actions.
• Add S3-compatible storage support through endpoint configuration, injected clients, requester pays, expected bucket owner, and provider documentation for R2, MinIO, Wasabi, DigitalOcean Spaces, Backblaze B2, path-style endpoints, S3 Express, and access points.
• Add the six release-gap features: cursor/page APIs, metadata enrichment, s3:// URI helpers, streaming reads, provider compatibility guidance, and advanced endpoint guidance.
• Preserve the focused niche: a small typed tree/object browser for app UIs and APIs, not a filesystem abstraction or CLI.

Validation

• PATH=/tmp/s3tree-modern-venv/bin:$PATH make lint typecheck test PYTHON=/tmp/s3tree-modern-venv/bin/python
• python -m build

Coverage is currently 100.00% total coverage with 50 tests passing, 0 missed statements, and 0 partial branches.