0.9.2: keep a rare two-valued column's bin in the WoE floor

[orgoca]

A patch: the WoE floor no longer ignores rare 0/1 columns. No change to the runtime, the exporters, the artifact schema, or the benchmark, whose features are all continuous.

The bug

fit_reference bins each feature with a shallow tree whose leaves must hold at least 2% of rows. A two-valued column whose rarer value fell below 2% couldn't be split, so it landed in one bin and carried no information.

This surfaced in the #76 gate experiment on Diabetes 130-US Hospitals. One-hot columns for rare discharge outcomes were lost, among them patients who died: 1.5% of rows, never readmitted. The same split, on all patients:

Floor, Report Gini
0.9.1	0.2353
0.9.1 with 0.5% bins (a workaround)	0.2958
0.9.2	0.2998
plain logistic regression on the same matrix, for scale	0.3105

This matters because compile_selected's floor gate compares against this model, and so does check 10 when given a fitted reference. A weakened floor let through artifacts a fair one would have stopped. 0.9.1 also advises adding 0/1 missing-indicator columns, which is exactly the kind of column the floor dropped.

The fix

A column with exactly two values is already binned, so it's cut between the two values, where the tree would, whatever the rarer one's share. The 2% minimum still governs the cut points a tree chooses on columns with more values, which is what it exists for. A 0/1 column the tree could already split gets the same cut as before.

Behaviour change: on data with rare two-valued columns, the floor gets stronger. A floor gate or check 10 that passed may now fail. That is the fix working, and the changelog says so.

Release

The second commit bumps to 0.9.2 (__init__, CITATION.cff, CHANGELOG). It also corrects 0.9.1's changelog date to its release day, 2026-10-10. The benchmark doesn't move; results.json stays as the 0.9.0 run.

Checks

• New test: a 1%-share 0/1 column gets two bins and a nonzero coefficient. It fails on 0.9.1 (no bins) and passes here. A common 0/1 column keeps its 0.5 cut.
• Full suite green locally; ruff and black clean.