Fix lint and types in scripts/cell_cost.py

[seanwevans]

The lint job on main is currently failing. scripts/cell_cost.py is neither black-clean nor mypy-clean:

$ black --check scripts/cell_cost.py
1 file would be reformatted.
$ mypy scripts/cell_cost.py
Found 7 errors in 1 file

This is my fault, from #292, and the way it slipped through is worth recording: pre-commit run --all-files only checks files git tracks. The script was still untracked when I ran the hooks before committing, so black and mypy never saw it and reported a clean run on a file they had skipped. I took that as verification. It wasn't.

The fixes

Two of the mypy errors were real defects, not missing annotations:

Values typed as object. build_report / measure_lifecycle / print_report declared dict[str, object], which makes every read of the report an error at the use site — and those errors were being silenced with per-line # type: ignore[index] / [union-attr] comments. They are heterogeneous mappings, so Any is the honest type, and the ignores go away with it.

A list of None. The sequential baseline was:

sequential = time_ms(lambda: [exec(CPU_WORK, {}) for _ in range(workers)])

building a list from a function that returns nothing. Replaced with a loop, which is what it meant.

The rest is black reformatting of three over-long lines.

Testing

• pre-commit run --all-files: isort, black, pylint, flake8, mypy all pass.
• tests/test_cell_cost.py: 6 passed.
• Ran the script on free-threaded 3.14 in both table and --json modes to confirm the behaviour is unchanged:

import surface           create+exec     close    RSS/cell
                              p50 ms    p50 ms         KiB
bare                           11.30      4.03        3662
json/re/dataclasses            31.45      8.37        7754
typical stdlib                 62.40     15.16       13584
dispatch onto warm cell         0.847
fork() of warm parent          1.229
parallel scaling at 4 workers:  threads 3.16x   cells 3.78x

Worth merging before anything else in flight, since every other PR's lint job inherits this failure.

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https://claude.ai/code/session_01Fs1hTtmF4Hm9h617AG9Gse

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