Rename Rust signer to engine

[hoffmang9]

Summary

• Rename the Rust signer crates, PyO3 package, CLI, and Python bridge/protocol surfaces to GreenFloor engine naming.
• Update CI, docs, tests, and build hints to use greenfloor-engine / greenfloor_engine.
• Add a repo-local Cargo target directory so GreenFloor builds do not write build output into chia-wallet-sdk.

Test plan

• pre-commit run --all-files
• cargo check --manifest-path greenfloor-engine/Cargo.toml
• cargo check --manifest-path greenfloor-engine-pyo3/Cargo.toml
• .venv/bin/python -m pytest -q

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