fix(python-sdk): pass extension-module link args outside maturin

[locez]

Problem

Building merry-py with plain Cargo (cargo build, cargo test) cannot link the _merry cdylib on macOS. With the pyo3/extension-module feature PyO3 stops linking libpython, so the module needs -undefined dynamic_lookup to let the host interpreter resolve the Python symbols at import time. Only a maturin build added those arguments, so plain Cargo could not produce a loadable module.

Change

• Add crates/merry-py/build.rs, which calls pyo3_build_config::add_extension_module_link_args(), the PyO3-owned helper that emits the required cargo:rustc-cdylib-link-arg values.
• Pin pyo3-build-config in [workspace.dependencies] to the same release as pyo3 and use it as a build dependency of merry-py.

The helper is a no-op where those arguments are not needed, so Linux and Windows link output is unchanged, and there is no production dependency, runtime state, or Python-visible behavior change. The extension-module contract stays with PyO3 instead of being re-hardcoded in the workspace.

Verification

• cargo fmt --all --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all
• cargo build -p merry-py links the cdylib on Linux with the build script active

macOS linking was not exercised in this environment. The Python SDK workflow matrix covers macOS, where maturin develop already supplied the same arguments, so the new path is exercised by plain cargo build/cargo test on a macOS checkout.