build(deps): consolidate upgrades and fix compatibility

[bardhh]

Update the dependency pins and uv lockfile together, including the matched MuJoCo/MJX 3.13.0 pair, mypy 2.3.1, Ruff 0.16.8, PyYAML stubs, pytest 9.1.1, tqdm 4.70.1, and kvxopt 1.3.3.2. Upgrade artifact uploads to v7 in both workflows.

The new kvxopt stubs expose incompatible assumptions in the platform-specific solver imports. Resolve that optional backend through a module boundary, preserve the missing-extra error, and cover ARM/x86 package selection when the backend is absent. The wheel smoke test now installs the wheel's declared MuJoCo extra instead of a separate stale 3.11.0 pin.

Supersedes #384, #385, #386, #387, #388, #389, #392, #393, #394, #395, and #396.

Validation:

• Python 3.12 core suite: 755 passed, 11 skipped.
• Python 3.10 examples/slow suite: 67 passed; optional Torch modules skipped at collection.
• Python 3.10 MuJoCo suite: 206 passed, 3 optional oracle tests skipped, 28 high-memory G1 tests excluded.
• Solver and missing-backend checks: 8 passed on Python 3.12; numerical checks and missing-backend checks also passed on Python 3.10.
• Ruff, Black, isort, and mypy passed across the source tree.
• Lockfile consistency, wheel/sdist metadata, and installed-wheel smoke tests passed. The fresh wheel environment resolved both MuJoCo packages to 3.13.0.