Union run containers by run instead of by value

[gitRasheed]

Description

runContainer16.inplaceUnion added the other container one value at a time. An in-place Or on run containers got slower with every value. It now inserts up to 16 runs in place and merges above that, like inplaceIntersect does.

Type of Change

• Performance improvement
• Test improvements

Changes Made

What was changed?

• inplaceUnion inserts runs in place or merges.
• union copies runs in blocks.
• toArrayContainer allocates once.
• iorRun16 merges above 4 runs.
• The union and intersect helpers check the next run before searching.
• New union tests and three real-data benchmarks.

Why was it changed?

An in-place Or over every adjacent pair in weather_sept_85_srt took 85 ms.

How was it changed?

A few runs are cheaper to insert than a full merge. Many runs are cheaper to merge. Without the in-place path, folding small bitmaps into a large accumulator was 2.3x slower than master on c7i.

Testing

go test ./... passes on c7i.xlarge and c8g.xlarge. The root package also passes with -tags=gofuzz.

Formatting

go fmt, make unconvert and git diff --check are clean.

Fuzzing

smat, 300 seconds: 1.5M executions, no failures.

Performance Impact

Real datasets after RunOptimize, less time than master (geometric mean):

Instance	Or in place	Or fold	And in place
c7i.xlarge	72%	85%	6%
c8g.xlarge	73%	85%	6%

Sorted datasets gain the most. The in-place pass over weather_sept_85_srt drops from 85 ms to 3.0 ms.

Benchmark	c7i master	c7i this PR	c8g master	c8g this PR
RunContainerInplaceUnion, 4 runs	3.73 µs	1.92 µs	4.02 µs	2.30 µs
RunContainerInplaceUnion, 1,024 runs	665.8 µs	19.6 µs	637.1 µs	19.4 µs
ArrayContainerIorRun16, 4 runs	2.83 µs	2.83 µs	2.76 µs	2.77 µs
ArrayContainerIorRun16, 100 runs	61.5 µs	4.75 µs	57.8 µs	3.75 µs

Some small shapes are slower, mostly operands whose runs the receiver already holds. They cost a few µs more, up to 7.6 µs instead of 3.8 µs on c7i. Skipping runs the receiver already contains before merging would avoid this. I left it out to keep the PR small.

Breaking Changes

None.