fix(ci): clear ruff failures on dev post-#96 merge

[Knapp-Kevin]

Summary

Fast-follow lint hygiene PR after PR #96 (preflight eval phase 3 — real-I/O C2/C3) merged with ruff failures still on its HEAD. Dev's CI Phase 1 `ruff + mypy` check is currently red on `5f773e6`; this PR clears it.

Why

Timeline:

1. PR Preflight eval phase 3 — real-I/O C2/C3 measurement (Jin's review feedback) #96 was retargeted from main → dev and rebased to resolve 19 conflicts.
2. The ruff+mypy CI Phase 1 gate (PR chore: CI Phase 1 — Windows matrix + ruff/mypy + secret scan + merged-to-dev labeller #102) flagged 8 lint issues on the merged SHA `6bf4a91`.
3. I pushed lint fixes to PR Preflight eval phase 3 — real-I/O C2/C3 measurement (Jin's review feedback) #96's branch (`f052421`, `956d5ef`, `e111860`) but the squash-merge had already executed at `6bf4a91` before those landed.
4. Dev now has the lint issues, blocking every subsequent PR's CI.

This PR re-applies the same fixes directly against current dev HEAD.

What changed

File	Fix
`tests/eval/_baseline_io.py`	ruff format (4 lines)
`tests/eval/run_preflight_cost_eval.py`	ruff format (line wrapping)
`tests/eval/test_cost_baseline_helpers.py`	I001 import sort + format
`tests/eval/_seed_ledger.py`	ruff format
`tests/test_ephemeral_authoritative.py`	F541 — `f"..."` with no placeholder → `"..."`

8 auto-fixes total, 5 files reformatted, zero behavioural changes.

Linked

Refs #96 (the merge that introduced the regression), #102 (the CI gate that caught it).

Test plan

• `ruff check tests/eval/ tests/test_ephemeral_authoritative.py` — all checks passed
• `ruff format --check` — clean
• `pytest tests/eval/test_cost_baseline_helpers.py -q` — 35/35 passed (unchanged behaviour)

Plan / Audit / Seal

Plan: trivial; risk:L1 (lint hygiene only; no behavioural code paths).

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