sl0ppty: ruff formats the Python, and sorts its imports

The other half of the pre-commit hook. C goes through clang-format, Python through
ruff: `ruff check --select I --fix` to sort the imports, then `ruff format` to lay
them out. `make fmt` does both, `make fmt-check` reports, `make hooks` installs it.

ruff rather than black because it is one static binary instead of a Python
dependency chain, and fast enough that a hook is free. Its formatter is
black-compatible, so this is the ecosystem's formatting rather than a dialect --
measured at 9,935 changed lines against black's 9,953, which is the same answer.

**Left entirely at the defaults**, which is the opposite of what `.clang-format`
does. That file argues with LLVM in five places because the C here was
hand-formatted into a house style worth keeping. The Python was not, and a
formatter of this kind is only worth having when there is nothing left to argue
about: every knob turned is an argument somebody has to have again later. 88
columns, double quotes, ruff's own line breaking.

**Import sorting is on, and C include sorting stays off.** Not an inconsistency: a
C include order can be load-bearing -- `_GNU_SOURCE` ahead of the system headers,
and a grouping somebody wrote by hand -- while Python import order is not. Where it
genuinely is, a statement between imports ends the block ruff will sort, and the
test files lean on exactly that:

    import sys
    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
    from harness import Session, check, report

`from harness import` cannot be hoisted above the line that makes it importable,
because the `sys.path.insert` between them is a barrier. Verified before touching
anything, since getting it wrong would have broken every test in the suite at once.

Two details in the hook worth naming. `--select I` is passed on the command line as
well as set in `ruff.toml`, so a stray `select` in somebody's own config cannot
turn a formatting hook into a linter that refuses a commit over an unused variable.
And the two tools are named together when either is missing, so setting up a
machine installs both at once instead of finding the second one on the next commit.

`set -e` and `[ -n "$c_files" ] && clang-format ...` do not mix: when the list is
empty the AND-list returns non-zero and the shell exits, so a Python-only commit
would have aborted with no message at all. They are `if` blocks. Found by testing
the case rather than by reading it.

Verified in a scratch clone, seven paths: Python alone, C alone, both in one
commit, imports sorted with the `sys.path` barrier intact, a `contrib/` script that
is Python with no extension on it, a partly staged Python file refused with `HEAD`
unchanged, and a commit with no code in it drawing no comment.

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