Native regr_* aggregates return wrong results for a constant variable merged from several partitions #6423

Describe the bug

regr_slope, regr_intercept, regr_r2, regr_sxx, regr_syy and regr_sxy run natively by default since #4775. The bug needs two conditions:

• a group's variable is constant at a value that binary floating point can't represent exactly, such as 0.1
• the group's rows are merged from two or more partial aggregates

When both hold, Comet returns plausible-looking wrong values where Spark returns NULL or 0.0. In 1.0.0 these aggregates ran in Spark, so this is a regression in 1.1.0. It reproduces on 1.1.0-rc1 and on main.

Steps to reproduce

// In a suite extending CometTestBase, default configs
spark.range(0, 6, 1, 2)
  .selectExpr("CAST(id AS DOUBLE) AS y", "0.1D AS x")
  .write
  .parquet(path) // two files, so two scan partitions
withParquetTable(path, "t") {
  checkSparkAnswer(
    "SELECT regr_slope(y, x), regr_intercept(y, x), regr_r2(y, x), regr_sxx(y, x), " +
      "regr_sxy(y, x), regr_r2(x, y), regr_syy(x, y) FROM t")
}

Expected behavior

Spark and Comet 1.0.0 return NULL, NULL, NULL, 0.0, 0.0, 1.0, 0.0.

Comet 1.1.0-rc1, with CometHashAggregate in both the partial and the final stage, returns 2.16e17, -2.16e16, 0.7714, 2.89e-34, 6.2e-17, 0.7714, 2.89e-34 (Spark 4.1 profile).

Additional context

The merge in native/spark-expr/src/agg_funcs/welford.rs (variance_merge, covariance_merge) uses a different floating-point operation order from Spark's CentralMomentAgg and Covariance. The first merge into the zero-initialized final buffer leaves a one-ULP error on the mean. As a result m2 ends up around 1e-34 instead of exactly 0, and Spark's exact m2 == 0 checks for the degenerate cases never fire.

The same merge order already makes multi-partition var_pop and stddev_pop drift on clustered values in 1.0.0. The tests in #4775 use whole-number constants and a tolerance, which hide it.

For 1.1.0 the smallest fix is to mark the regr_* aggregates Incompatible, so they fall back to Spark as they did in 1.0.0. Porting Spark's merge order is the full fix, and it would also fix var_pop and stddev_pop.

Found by the 1.1.0 regression audit (#6399) and tracked in #6402.