SPARK-1699: Python relative independent, becomes a subproject - #631

The current doc hints spark doesn't support accumulators of type `Long`, which is wrong.

JIRA: https://spark-project.atlassian.net/browse/SPARK-1117

Author: Xiangrui Meng <[email protected]>

Closes apache#631 from mengxr/acc and squashes the following commits:

45ecd25 [Xiangrui Meng] update accumulator docs
(cherry picked from commit aaec7d4)

Signed-off-by: Patrick Wendell <[email protected]>

Cherry pick SPARK-27291 and SPARK-27990

### What changes were proposed in this pull request?

`dev/varka_vector_bits.java`, the probe the gate runs to print the narrow step's vector width (VARKA-286, apache#629), moves to `dev/varka_canary/VectorBits.java`, beside the other JVM probes. `dev/varka_gate.sh` runs it from there.

**Why it broke MiMa.** Spark's sbt build defines the project `oldDeps` with `dev/` as its base directory (`project/SparkBuild.scala`, `Project("oldDeps", file("dev"))`). `dev/mima` builds that project, and sbt compiles any Java file sitting directly in a project's base directory. The probe imports `jdk.incubator.vector`, which that project doesn't have, so the step fails with "Failed to export oldDeps classpath". sbt doesn't compile subdirectories of the base, so `dev/varka_canary/` is safe.

**Why apache#629 was green.** Pull requests that change only Varka's files skip Spark's linter job, MiMa included. The failure shows up only on pull requests that run Spark's full checks, such as apache#631, which changes the workflow; it has failed MiMa twice.

### Why are the changes needed?

So that MiMa passes again on every pull request that runs it, starting with apache#631.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

`dev/mima` passes locally on this branch; on apache#631's branch it fails at the probe ("package jdk.incubator.vector.IntVector does not exist"). The moved probe prints 128 with `-XX:MaxVectorSize=16` and 512 without, as before, and `bash -n dev/varka_gate.sh` is clean.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Claude Opus 5.5)