MAINT: pin ruff rule set, lint whole repo, fix sum_spectra NameError

[physicien]

Why

CI broke on Dependabot PR #7 (actions/setup-python v6 → v7). The bump was innocent: its whole diff is one line, and the failure was in the ruff check step.

The real cause is ruff version drift. CI installs ruff through the unpinned ruff>=0.4 extra and ran it with no configuration, so the rule set was whatever ruff defaulted to that day:

run	ruff	result
2026-07-08 (PRs #5, #6)	0.15.20	pass
2026-08-01 (PR #7)	0.16.1	30 errors

Same sources in both. main was equally broken, not just the PR.

What changed

• pyproject.toml: select = ["E", "F", "W", "I"] and line-length = 80, matching the project convention. The rule set is now ours rather than ruff's shifting default, so a future release cannot reopen this. The 26 UP/SIM/RUF findings that broke CI are out of scope by construction, not suppressed. The longest line in src/ was already exactly 80, so the limit costs nothing.
• Imports sorted across src/, tests/, tools/; 19 over-long test lines wrapped; two unused test imports dropped. No assertion or test logic changed.
• ruff check src/ → ruff check ., so tests/ and tools/ are linted too.
• Bug fix, tools/sum_spectra.py: the except IOError handler referenced filename, which is not defined (the loop variable is path; args.filename is the argparse list). A missing input file raised NameError from the handler instead of reporting the file, leaving sys.exit(1) unreachable. Invisible to CI because src/ was the only linted path.

Verification

Run with ruff 0.16.1, the version that broke CI:

• ruff check . → passes on each of the three commits, against the lint command that commit's own ci.yml specifies.
• pytest tests/ → 134 passed, identical to the pre-change baseline rebuilt on the unmodified tree, so the import reordering moved nothing.
• sum_spectra.py exercised both paths: missing file now prints 'no_such_file.out' not found and exits 1; summing two copies of an ESD spectrum gives exactly 2×, and the output header and grid are unchanged.

Verification environment was a throwaway venv (Python 3.13.5, numpy 2.5.2, pandas 3.0.5, matplotlib 3.11.1, scipy 1.18.0), newer than the requirements/ pins, so this is "passes on current upstream" rather than a check against the pinned build.

Not addressed

CI still installs unpinned ruff>=0.4 while requirements/requirements-development.txt pins ruff==0.15.20. The rule-set pin makes ruff's defaults irrelevant, but a future release could still change behaviour inside E/F/W/I.

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