chore: simplify redundant dict comprehensions (ruff C416, C420)

[alexzhu0]

Summary

Two small ruff-flagged idiom cleanups from the flake8-comprehensions family:

• C420 — {k: v for k in it} → dict.fromkeys(it, v) (4 sites)
• C416 — {k: v for k, v in zip(a, b)} → dict(zip(a, b)) (4 sites)

Same behavior, slightly faster, and more obvious intent at a glance. After this PR, ruff check --select C416,C420 m_flow/ is clean.

Files

File	Rule	Notes
m_flow/eval/report.py	C420	FAILURE_BUCKETS zero-init
m_flow/ingestion/documents/classify_documents.py	C420 ×3	office/image/audio ext → doc-type maps
m_flow/retrieval/episodic/bundle_search.py	C416	collection→distances map
m_flow/retrieval/utils/fine_grained_triplet_search.py	C416	collection→distances map
m_flow/retrieval/utils/procedural_bundle_search.py	C416	collection→distances map
m_flow/tests/test_relational_db_migration.py	C416	test helper

Verification

$ ruff check m_flow/ --exclude 'm_flow/baml_client' --select C416,C420
All checks passed!

$ python -c "from m_flow.ingestion.documents.classify_documents import _IMAGE_EXTS, _OFFICE_EXTS, _AUDIO_EXTS; \
print(len(_IMAGE_EXTS), len(_OFFICE_EXTS), len(_AUDIO_EXTS))"
16 9 9

No behavior change. No new tests needed — these rules flag exact semantic equivalents.

I affirm that all code in every commit of this pull request conforms to the terms of the M-flow Developer Certificate of Origin

[FlowElement-ai]

Thanks for this cleanup! Two small things hold this back from merging:

1. DCO: I added the affirmation to the PR body for you, the Verify DCO statement check should pass now.
2. Fork PR lightweight checks still fails because m_flow/ingestion/documents/classify_documents.py (the file your dict.fromkeys autofix touches) is unformatted under the repo-pinned ruff 0.13.1 in uv.lock. Newer ruff joins the 16-element image-extension list into one line (>120 chars), but 0.13.1 wants the list re-expanded.

To unblock, please run locally and push:

uv sync --group dev
uv run ruff format m_flow/ingestion/documents/classify_documents.py
git commit --amend --no-edit && git push --force-with-lease

Same idea applies to your other open PRs that 're hitting the same fork-lite check. After that the queue should go green and we can merge the batch. Thanks!

[FlowElement-ai]

Quick update: I just merged #142 which bumps the lockfile ruff from 0.13.1 → 0.15.10 to match the pyproject.toml ceiling. That was the version that disagreed with your autofix output on every recent fork PR.

Triggered a fork-lite rerun on this PR with the new lock — it still failed, which means your ruff is even newer than 0.15.10. The smallest unblock is one of:

# either downgrade locally to match the repo
uv tool install ruff@0.15.10
ruff format .

# or just re-run with whatever ruff comes from the project venv
uv run ruff format .

git add -u && git commit --amend --no-edit && git push --force-with-lease

After that the fork-lite check should go green and we can merge. Sorry for the back and forth on this one — the lockfile drift was the underlying cause.