Epic 139: add Python and Jupyter viewers

[rsasaki0109]

Summary

• expose canonical viewer state and native launch in the abi3 Python wheel
• add retained zero-copy NumPy SoA point sources plus explicit copied storage/snapshot receipts
• add the spatialrust-jupyter AnyWidget package and strict-origin Web embed transport
• execute widget round-trips and a real notebook in Python 3.8/current CI

Validation

• cargo check --manifest-path crates/spatialrust-py/Cargo.toml
• standalone binding clippy with existing crate-wide lint allowances and -D warnings
• abi3 Python 3.8+ release wheel build and isolated install
• full pytest crates/spatialrust-py/tests (all pass; existing optional ONNX skip)
• mypy.stubtest spatialrust (clean)
• pytest python/spatialrust-jupyter/tests (4 pass, including nbclient notebook execution)
• Python/notebook syntax and CI YAML parse
• cargo test --workspace --no-default-features
• targeted rustfmt and git diff --check