Release einfold for Python 0.2.0rc2 - #144

🪭 Claude (einfold)

This sets einfold for Python's version to 0.2.0rc2. Once it merges and python-v0.2.0rc2 is tagged on it, the release candidate goes to PyPI. Users then get it with pip install --upgrade --pre "einfold[datafusion]", and the plain install keeps 0.1.0. It's for xarray-sql's acceptance testing. xarray-sql is an XQL Systems project that queries Xarray datasets with SQL. xarray-sql passed 0.2.0rc1 (xqlsystems/xarray-sql#285).

Since rc1:

• Contract chains of three products in the cheaper order, under a float guard #143 computes a chain of three products, such as SUM(a.v * b.v * c.v) over three joined tables, as two dense products in the cheaper order. It reorders only where a float guard shows no sum can overflow, and otherwise runs Find products whose factors read three or more relations, as folds of regrouped operands #131's fold or the host's plan. A chain of matrix products is now 3× DataFusion at n = 32 (it tied in rc1), 22× at 64, and 450× at 128. DuckDB is slower than DataFusion at every size measured.
• Plan functions with the session's configuration in the Python bridge #140 fixes The Python bridge runs functions with DataFusion's default configuration, so to_unixtime ignores the session's time zone #136: from Python, functions now run with the session's configuration, so to_unixtime reads text in the session's time zone. 0.1.0 and rc1 read it in UTC.
• Test that einfold changes no built-in function's answer from Python #141 checks that einfold changes no built-in function's answer from Python, under two time zones.
• Simulate fills, three-relation products and float division #134 and Pair rows that differ only in a zero's sign alike in both runs #142 extend the simulation, einfold's random comparison against DataFusion, to rc1's new shapes, and fix its pairing of rows that differ only in a zero's sign (Nightly fuzz: 14 property failure(s) in the simulation soaks #138).

Still open for M3: a chain whose last factor is a vector is 0.8× DataFusion at n = 32 (4.1 ms against 3.2 ms), and TPC-H is 1.06× DataFusion's time. Both are the Python bridge's fixed cost per query (#81, #58). The fused backward pass waits on xqlsystems/ddx#243.

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