feat(report): make the effect column selectable (--effect-col)

[dchaudhari7177]

Closes #30.

Problem

report.py pinned the effect column at module scope (_COL_EFFECT = "formality") and _read_sweep_rows read row[_COL_EFFECT] directly. A sweep for either of the other two shipped concepts — sentiment, verbosity — could not be reported at all: the missing-columns guard rejected the CSV before parsing, and the only workaround was editing the module.

This already bites the committed artifacts. 14 of the 24 sweep CSVs in artifacts/ cannot be rendered on main — the cross-model sentiment/verbosity runs and the redosed layer sweeps emit the effect column as effect rather than formality:

$ python -c "from pathlib import Path; from steerbench import report; \
    report.load_dose_curve(Path('artifacts/dose_response_sentiment_qwen.csv'))"
ValueError: artifacts/dose_response_sentiment_qwen.csv is missing columns: ['formality']

With this PR, steer-report --dose-csv artifacts/dose_response_sentiment_qwen.csv ... --effect-col effect renders them.

Change

effect_column is threaded through the read path and defaulted to "formality" everywhere, so nothing about current behaviour changes:

• _read_sweep_rows(path, x_column, effect_column="formality")
• load_dose_curve(path, effect_column="formality")
• load_layer_curve(path, x_column="layer", effect_column="formality")
• build_report(..., effect_column="formality")
• steer-report --effect-col NAME (default formality), passed straight through

_COL_EFFECT becomes DEFAULT_EFFECT_COLUMN (the CLI needs it to build its --help text and default), and _SWEEP_COLUMNS now holds only the columns that are the same for every concept — seed, repetition, ppl — with the x column and the effect column added per call.

Error path

The requested column is folded into the existing missing-columns check rather than getting a second guard, so a wrong name fails before any row is parsed instead of raising a bare KeyError mid-file. The message now also lists what the CSV does carry:

dose.csv is missing columns: ['verbosity'] (available: ['alpha_norm', 'coeff', 'formality', 'ppl', 'repetition', 'seed'])

Tests

CPU-only, no model and no download — the fixtures are the existing canned CSVs with the header renamed.

• test_effect_column_is_selectable — a sentiment dose CSV and layer CSV parse correctly with effect_column="sentiment", and the coherence columns are unaffected.
• test_effect_column_defaults_to_formality — the same sentiment CSV still fails the guard on the default, so the default really is unchanged.
• test_unknown_effect_column_names_the_available_columns — asserts the requested name, the word available, and the real column all appear in the message.
• test_cli_effect_col_reports_a_non_formality_sweep — end to end through steer-report: the committed M0 artifacts with the header renamed formality → sentiment render byte-identical markdown to the unrenamed run under the default.
• test_cli_effect_col_unknown_column_is_reported — the CLI surfaces the named ValueError.

Checks

ruff check ., ruff format --check ., mypy src, pytest -q all pass (68 passed).

Two notes on running the suite on Windows, neither caused by this change:

• mypy src reports a syntax error inside numpy/__init__.pyi if numpy happens to be in the environment (it comes in with the optional report extra, not with the dev group) because the config targets python_version = "3.11". mypy --python-version 3.12 src is clean; CI does not install numpy, so it is clean there too.
• The three tests/test_cli.py tests that actually render a card fail on Windows under a non-UTF-8 locale, on clean main as well as here: build_report calls Path.write_text with no encoding=, so the Δ in the side-effects table and the ⚠️ in the trap warnings hit cp1252. I verified this by stashing my changes and re-running. It is a separate bug and I'll send a separate PR for it rather than fold an unrelated fix in here; with PYTHONUTF8=1 the full suite is green locally.

Deliberately out of scope

The rendered card still labels the series generically ("effect (behaviour score)") rather than naming the concept. Doing that properly means carrying the column name into ReportData and through both renderers and both plots, which is a wider change than this issue asks for — happy to follow up if you want it.