fix: decide reference percent-vs-fraction scale per column, not table-wide

[propcgamer20-png]

Problem

faircode/profiler.py's parse_reference (and the JS twin parseReference in assets/profiler-engine.js) made the percent-vs-fraction decision once, globally, across the entire reference file:

scale = 100.0 if any(s > 1.5 for _, _, s in raw) else 1.0

A reference file assembled from multiple sources - one column already given as fractions, another as percentages - gets the wrong scale applied to whichever column didn't trigger the heuristic.

Repro (before)

ref_mixed.csv with sex as fractions and race as percentages:

column,group,share
sex,Female,0.6
sex,Male,0.4
race,White,70
race,Black,20
race,Other,10

race's 70 trips the 1.5 threshold, so scale = 100 is applied table-wide, dividing sex's correct 0.6/0.4 down to 0.006/0.004. The sex reference comparison becomes nonsense (0.4-0.6% expected vs 50% actual).

Fix

Group rows by the column identifier and decide the scale per column:

by_col = {}
for col, grp, share in raw:
    by_col.setdefault(col, []).append((grp, share))
reference = {}
for col, pairs in by_col.items():
    scale = 100.0 if any(s > 1.5 for _, s in pairs) else 1.0
    for grp, share in pairs:
        reference.setdefault(col, {})[grp] = share / scale

Same change mirrored in assets/profiler-engine.js (Python/JS parity), and SPEC.md section 9 updated. Single-convention files parse exactly as before.

After

parse_reference(ref_mixed) -> {"sex": {"Female": 0.6, "Male": 0.4}, "race": {"White": 0.7, "Black": 0.2, "Other": 0.1}}

Tests

• test_parse_reference_mixed_scale_is_decided_per_column (test_profiler.py)
• test_python_js_parse_reference_mixed_scale_parity (test_js_parity.py) - runs parseReference under Node and asserts byte-identical result to the Python engine

pytest tests/test_profiler.py tests/test_js_parity.py -> 61 passed, 1 failed. The failure (test_python_js_profiler_parity_sniffs_quoted_newlines) is pre-existing on main on this platform and unrelated. ruff clean.

Closes #513

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