Add micropip runtime support

[zklapow]

Summary

• add host-side MicropipResolver support and a default HTTP resolver
• add allowlisted PyPI resolver helpers that parse requirements or lock-style files and reject non-allowlisted requests before making HTTP requests
• route remote package bytes through HostBridge with chunked transfer
• source the patched micropip runtime package from the zklapow/micropip boomslang branch via a submodule
• install the submodule package into generated Python runtime resources through just resources and reapply it after just fetch-main-wasm installs published main runtime assets
• document asyncio.run usage and pure-Python wheel limits

Validation

• nix develop -c just fetch-main-wasm
• nix develop -c ./mill --jobs 0 build
• nix develop -c ./mill --jobs 0 test
• nix develop -c just python-test
• nix develop -c just python-wheel
• installed boomslang-py/dist/*.whl into a fresh venv and ran boomslang-py/tests