Replace the term Plugin with Extension

[schaubh]

Summary

Rename BSK-SDK’s out-of-tree wheel terminology from “plugin” to “extension” while preserving the existing public CMake API for downstream users.

This updates the documentation, example project, CI labels/artifacts, CMake diagnostics/comments, and tests to use extension naming. The example project is renamed from examples/custom-atm-plugin to examples/custom-atm-extension, and its wheel/project metadata now uses bsk-extension-exponential-atmosphere.

This also prepares the SDK for the Basilisk 2.12 beta cycle by syncing the SDK version to 2.12.0b0, teaching CI to recognize PEP 440 alpha/beta versions such as 2.12.0a1 and 2.12.0b0, and making tools/sync_all.py update both src/bsk_sdk/_bsk_version.txt and the [project].version field in pyproject.toml.

Affected Surface

• Packaging: pyproject.toml now reports 2.12.0b0
• CMake/SWIG: comments and diagnostics now refer to extensions
• Sync tooling: tools/sync_all.py stamps both SDK version files
• Example extension: examples/custom-atm-extension
• CI: beta/alpha SDK versions route to BSK develop and nightly wheels
• Docs/tests: terminology and smoke assertions updated

Basilisk Version

Synced SDK version: 2.12.0b0

This branch is intended for the BSK 2.12 beta cycle. For aN / bN versions, CI tests against Basilisk develop and the nightly bsk[all] package set.

Downstream Compatibility

Existing BSK-SDK 2.10-2.11 users do not need to rename their projects. The public CMake helpers remain unchanged:

find_package(bsk-sdk CONFIG REQUIRED)
bsk_add_swig_module(...)
bsk_generate_messages(...)

Users will still need to rebuild extension wheels against the matching BSK/BSK-SDK 2.12 pair, but this naming change should not require source changes in existing plugin-named projects.

Verification

Local checks run:

.venv/bin/pytest tests/test_sync_all.py -v

Earlier local compatibility checks were also run for the renamed example extension:

.venv/bin/pytest tests/test_smoke.py -v
python -m build --wheel --no-isolation -o /private/tmp/bsk-extension-dist examples/custom-atm-extension
python -m pip install --force-reinstall --no-deps /private/tmp/bsk-extension-dist/bsk_extension_exponential_atmosphere-0.1.0-*.whl
.venv/bin/pytest examples/custom-atm-extension/customExponentialAtmosphere/_UnitTest/test_customExponentialAtmosphere.py -v

[ReeceHumphreys]

Looks great, think this was the right direction!