What

• Re-export get_platform from the top-level pythonnative package.

Why

• get_platform() in pythonnative.platform is documented and unit-tested, but it was never added to pythonnative/__init__.py, so pn.get_platform() raised AttributeError. This makes the documented call work.

How (brief)

• src/pythonnative/__init__.py: import get_platform alongside Platform and add "get_platform" to __all__.
• scripts/check-e2e-coverage.py: add get_platform to INTENTIONAL_EXEMPTIONS (thin functional alias of Platform.OS, no device surface).
• tests/test_platform.py: add a test asserting pn.get_platform() == pn.Platform.OS through the package surface.

Testing

• ./scripts/check.sh passes; python scripts/check-e2e-coverage.py exits 0.

Risks/Impact

• Additive only. New public name, no behavior change to existing symbols.

Docs/Follow-ups

• None; existing docs already reference pn.get_platform().

Closes #44