fix(code-quality): black-version default 23.1.0 → 26.3.1

[LukasWodka]

The 23.1.0 default I shipped in #115 rested on a measurement that was itself the bug.

black 23.1.0 cannot run on Python 3.12+ (it calls ast.Str, removed in 3.12): it errors on every file with error:/would fail to reformat while printing zerowould reformat lines. Grepping only the latter turned "black crashed" into a confident "0 dirty — pure version skew, no backlog".

Re-measured on Python 3.11 (zero parse errors, so real verdicts):

repo	black 23.1.0	black 26.3.1
backend	26	0 ← its own gate's pin
tracebloc-engine	148	157
tracebloc-py-package	125	168
data-ingestors	78	78
averaging-service	49	71
client-runtime	14	15

No repo is 23.1.0-clean. 26.3.1 is what backend is clean under, what the pre-commit hooks pin (engine, data-ingestors), and it runs on current Python. Callers should still pin explicitly — the doc string now says why.

Part of tracebloc/backend#1382

🤖 Generated with Claude Code

---

Note

Low Risk
Workflow default and documentation only; callers can still override black-version, with no auth or runtime product impact.

Overview
Bumps the shared black-version default from 23.1.0 to 26.3.1 in .github/workflows/code-quality.yml for repos that opt into the format job without overriding the pin.

The black-version input description is rewritten: it no longer claims fleet-wide cleanliness under 23.1.0, and instead tells callers to pin black in sync with pre-commit. It documents that 23.1.0 breaks on Python 3.12+ (ast.Str), can error on every file without would reformat lines, and that 26.3.1 matches backend’s gate and org pre-commit pins while running on current Python.

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[LukasWodka]

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