perf: avoid full-width post-clean narrative profiling

[WilliamK112]

Summary

• restrict explain_clean()'s post-clean context build to columns that can actually contribute a narrative
• skip the post-clean context build entirely when there are no rationale-bearing actions or remaining missing values
• retain full profiling for the pre-clean role table and the pipeline's separate post-representation engine cache, avoiding stale context reuse
• add a 120-column regression test and an Unreleased changelog entry

Why

explain_clean() unconditionally profiled every cleaned column even though _narratives() only reads contexts for columns with rationale-bearing actions or for columns that remain missing. On a wide no-op frame, that doubled per-column context construction from 120 to 240 calls. Reusing the pipeline cache directly would be unsafe because it is built after representation repair and only covers engine-visible columns, while the role table describes the original frame.

The new selection preserves the existing narrative semantics while avoiding the redundant full-width pass.

Closes #32

Validation

• red-first regression: the new wide-frame test failed before the implementation with 240 context builds instead of 120
• pytest -m "not online and not large" — 4,052 passed, 6 skipped, 12 deselected; 93.16% coverage
• pytest --no-cov -q tests/test_explain.py — 14 passed
• ruff check src tests
• mypy src/freshdata — no issues in 201 source files
• git diff --check

AI assistance

I used OpenAI Codex to inspect the current profiling flow, implement the optimization, and run validation. I reviewed the final diff and test evidence before submission.

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