[improvement](parquet) Decode fragmented dictionary indices in one pass

[Gabriel39]

What problem does this PR solve?

File Scanner V2 can produce hundreds of one-row selection ranges after compound raw filtering. Dictionary decoding then repeatedly enters skip, decode, and validation for every range. The roughly 5% fragmented Decimal predicate shape in TPC-DS Q28 misses the existing dense-selection threshold and spends significant CPU on dictionary-index transitions.

What is changed?

• Preserve the existing dense-selection policy.
• Decode dictionary IDs sequentially once when the estimated range-transition cost exceeds one full pass and the index scratch buffer fits in L2 cache.
• Keep clustered and very sparse selections on range decode.
• Add a 5% decoder benchmark axis and a regression test for the Q28-shaped selection.

Performance

Release build, same-host CPU-pinned A-B-B-A comparison, 9 repetitions per run. Values are average paired median CPU time; lower is better.

Scenario	Before (ns)	After (ns)	Change
Dictionary INT32, 5% alternating	191,996	25,664	-86.63%
Dictionary INT32, 10% alternating	375,928	34,112	-90.93%
Dictionary INT64, 5% alternating	234,943	31,844	-86.45%
Dictionary BYTE_ARRAY, 5% alternating	230,975	30,649	-86.73%
Dictionary INT32, 1% alternating (control)	68,290	44,670	-34.59%
Dictionary INT32, 5% clustered (control)	26,223	18,698	-28.70%
Plain INT32, 5% alternating (control)	11,344	10,781	-4.96%

No regressions were observed in the control matrix. Existing paths remain unchanged unless fragmented-selection transition cost crosses the new cache-aware threshold.

Validation

• 132 targeted Parquet native-decoder and benchmark-scenario tests passed.
• Release microbenchmark completed with the matrix above.
• Release BE build completed successfully.

This PR is stacked on #66485; the final commit contains this change.

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Codex automated review failed and did not complete.

Error: All Codex review accounts are usage-limited; earliest retry is 2026-08-08T03:32:00Z.
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[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Codex automated review failed and did not complete.

Error: All Codex review accounts are usage-limited; earliest retry is 2026-08-08T03:32:00Z.
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[hello-stephen]

BE UT Coverage Report

Increment line coverage 100.00% (16/16) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	58.27% (24674/42347)
Line Coverage	42.51% (248528/584601)
Region Coverage	38.55% (196233/508970)
Branch Coverage	39.95% (89832/224852)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (16/16) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.75% (30441/41275)
Line Coverage	57.68% (334695/580226)
Region Coverage	54.60% (278650/510336)
Branch Coverage	55.50% (124656/224610)

[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Codex automated review failed and did not complete.

Error: All Codex review accounts are usage-limited; earliest retry is 2026-08-08T03:32:00Z.
Workflow run: https://github.com/apache/doris/actions/runs/31064118892

Please trigger /review again after that time.

[hello-stephen]

BE UT Coverage Report

Increment line coverage 66.13% (248/375) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	58.35% (24745/42410)
Line Coverage	42.60% (249540/585763)
Region Coverage	38.63% (197036/510019)
Branch Coverage	40.03% (90258/225468)