Add doctest examples to the public SCS API and run them in CI

[tschm]

Closes #240

Every public symbol carried prose documentation but zero runnable examples, so nothing verified that what the docstrings claim is still true — the failure mode where a docstring keeps rendering perfectly long after the behaviour beneath it changed.

33 examples added:

Symbol	Examples
LinearSolver	3
SCS (new class docstring)	6
SCS.solve	9
SCS.update	8
solve (legacy)	7

Each is self-contained and deterministic — tight eps_abs/eps_rel, verbose=False, values rounded before printing — so they run standalone rather than depending on an earlier example's state.

This also fills two gaps in the same file: SCS had no class docstring at all, and the public legacy solve() had only a # comment above it rather than a docstring.

test/test_doctests.py runs them against the installed package (what users import) and asserts attempted > 0 as well as failed == 0, so a module that silently loses its examples fails rather than passes. I negative-tested that guard: injecting a wrong expected value gives attempted: 31 failed: 1.

The test takes capsys, which pytest-run-parallel auto-detects as thread-unsafe, so it serialises correctly under the free-threading job's --parallel-threads=4.

Full suite after the change: 397 passed, 67 skipped.

🤖 Generated with Claude Code

[bodono]

Thanks for the work, but as noted on #240 we're not adding doctests to the docstrings or a doctest job to CI. Closing.