chore+ci: bump version to 0.3.0 and pin official build toolchain

[KayMKM]

Release-window changes needed to build a correct v0.3.0 wheel.

chore(release): bump version to 0.3.0 (+ uv.lock sync)

Bumps pyproject.toml (and uv.lock) to 0.3.0. Version is exposed at runtime via importlib.metadata, so pyproject.toml is the only source.

ci(release): pin official build toolchain versions

ModelKit Official Build installed its toolchain with pip install --upgrade build twine packaging — completely unpinned. build 1.5.1 (released since v0.2.0) breaks the build inside the OneBranch container:

platformdirs\windows.py -> get_win_folder_from_registry
  winreg.QueryValueEx(key, CSIDL_LOCAL_APPDATA)
-> FileNotFoundError: [WinError 2] The system cannot find the file specified

The sdist step fails and the wheel step is skipped, so the build produces nothing. Verified on run 153739315 (failed, build 1.5.1) vs 150550609 (v0.2.0, succeeded, build 1.5.0). build 1.5.0 and 1.5.1 have identical _pip_env(), so this is environment behavior in 1.5.1, not a change on our side.

Pinning to the versions that produced v0.2.0 restores a green build and, more importantly, stops a third-party release from breaking the release build on publish day.

Validated: official build 153740145 succeeded on this branch — sdist, wheel, iKey verification, and PyPI validation all green.