test(diff): a worked example of the diff over two five-row tables

[paddymul]

What this adds

tests/test_diff_by_example.py: the diff of two five-row tables, written so that the code can be read and the result checked by eye. It changes no source.

The tables are joined on a_primary:

before                              after
a_primary  b_int  c_float           a_primary  c_float  d_string
1          100    1.5               1          1.5      one
2          200    2.5               2          5.0      two
3          300    3.5               3          3.0      three
10         1000   10.5              20         20.5     twenty
11         1100   11.5              21         21.5     twenty-one

Keys 1, 2 and 3 are in both: key 1 is identical, keys 2 and 3 differ in c_float. Keys 10 and 11 are only before, keys 20 and 21 only after. b_int is only in before and d_string only in after, so c_float is the one non-key column both sides have.

• test_the_diff_of_two_small_tables writes the whole compare table (build_compare_expr's result) out row by row and compares it with the real one. The membership and equality codes are named constants (c_float_eq is the membership, plus 4 when the values are equal), so BOTH + EQUAL reads as what it is.
• Four short tests state one claim each: which side each row came from, which of the three shared rows match, schema_diff naming b_int as removed and d_string as added, and d_string appearing in the diff.
• That last test is a strict xfail tied to a column added in the newer version never appears in the diff grid or a promoted diff: build_compare_expr and the column config loop over the a side only #275. build_compare_expr loops over the before side's columns, so d_string never reaches the compare table, and the expected table in the main test has no d_string column for the same reason. When a column added in the newer version never appears in the diff grid or a promoted diff: build_compare_expr and the column config loop over the a side only #275 is fixed the strict xfail fails, and the marker and the missing column can go.

No Buckaroo server, session or grid is involved. build_compare_expr imports one helper (_align_backends) from the buckaroo package, and tallyman_xorq.diff, which holds schema_diff, imports buckaroo.compare when it loads.

Verification

Local: tests/test_diff_by_example.py and tests/test_diff.py together: 45 passed, 1 xfailed. uvx ruff check: clean. The rest of the suite is left to CI.

The tests pass today, apart from the strict xfail, so there is no red commit.

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