fix: enforce Linux cp311 ABI3 release wheels

[parthdongre]

Summary

Hardens native-wheel packaging after the v0.3.0 public smoke exposed a Linux manylinux interpreter mismatch.

Root cause

maturin-action runs Linux manylinux builds inside its container. The release workflow passed --interpreter python, which resolved to CPython 3.9 inside that container. Maturin correctly detected the crate's abi3-py311 floor, warned that CPython 3.9 was too old for that ABI3 target, and emitted a version-specific cp39-cp39 Linux wheel instead.

Fix

• use python3.11 explicitly for the Linux manylinux release build;
• keep host python for macOS/Windows builds provisioned by actions/setup-python;
• fail each native release job unless its wheel is tagged cp311-abi3;
• fail the final release payload validation unless all four native wheels use cp311-abi3 and exactly one Linux x86_64 manylinux wheel is present;
• make the regular Package workflow assert its native artifact is also cp311-abi3 before running the pip-preference smoke.

A separate exact-v0.3.0 diagnostic build is validating the corrected Linux invocation on Python 3.12 before the missing Linux ABI3 file is added to the existing PyPI 0.3.0 release.

No runtime APIs or package version are changed.