feat(pwmj): support LeftSemi/LeftAnti existence joins via classic scan

[SubhamSinghal]

Which issue does this close?

Part of #17427 (Make PiecewiseMergeJoin work in DataFusion). Adds LeftSemi / LeftAnti support, one of the epic's checklist items. Supersedes the stale #18392, taking the alternative approach that @2010YOUY01 suggested there (reuse the classic join path for generality) rather than a dedicated existence stream.

Rationale for this change

An inequality-correlated EXISTS / NOT EXISTS (e.g. WHERE EXISTS (SELECT 1 FROM r WHERE l.x < r.y)) has no equi-key, so it decorrelates to a LeftSemi / LeftAnti join with a single range predicate. Today PiecewiseMergeJoinExec rejects existence joins (not_impl_err!) and these queries fall back to NestedLoopJoinExec, which is O(n*m).

Microbenchmark (20K × 20K rows, single inequality, enable_piecewise_merge_join
on vs off), added in this PR as piecewise_merge_join_semi_anti:

Case	PWMJ	NestedLoopJoin	Speedup
LeftSemi, high selectivity	~0.50 ms	~75 ms	~150×
LeftAnti, high selectivity	~0.44 ms	~75 ms	~170×
LeftSemi, low selectivity	~0.48 ms	~76 ms	~158×
LeftAnti, low selectivity	~0.48 ms	~75 ms	~155×

What changes are included in this PR?

Existence joins (LeftSemi / LeftAnti) for PiecewiseMergeJoin:

• Route LeftSemi / LeftAnti with a single range predicate to PiecewiseMergeJoinExec in the physical planner (still gated behind enable_piecewise_merge_join, default false).
• Reuse the classic scan: on the first match, mark the matching buffered-row suffix in the visited bitmap and emit from it in the final pass (LeftSemi = marked rows, LeftAnti = unmarked; NULL join keys are never marked, so they are correctly excluded from Semi and included in Anti). Only left-side columns are produced.
• RightSemi / RightAnti / Mark remain unsupported (they require swapping the inputs); they are still rejected in try_new and excluded in the planner. Left as a follow-up.
• Two small optimizations so existence marking stays O(buffered), not O(n*m): stop scanning a batch after the first match (the marked suffix is maximal), and a cross-batch low-water mark so later batches only mark not-yet-marked rows.
• Updated the operator docstring to describe the implemented classic-reuse path and keep the min/max fast-path as a documented follow-up.

Are these changes tested?

Yes.

• 33 unit tests in classic_join.rs covering LeftSemi / LeftAnti across <, <=, >, >=; NULL join keys; all-null streamed side; empty inputs; Date32 keys; multi-batch and multi-partition streamed inputs; and the low-water-mark skip branch.
• End-to-end SLT coverage in pwmj.slt for EXISTS / NOT EXISTS (including NULLs) with EXPLAIN assertions confirming the plan uses PiecewiseMergeJoin.
• The multi-partition test also guards the final-pass counter fix.

Are there any user-facing changes?

No behaviour change by default: enable_piecewise_merge_join remains false. When enabled, single-range-predicate LeftSemi / LeftAnti joins are planned as PiecewiseMergeJoin instead of NestedLoopJoin. No API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.37024% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.72%. Comparing base (592eeab) to head (5ec1790).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...lan/src/joins/piecewise_merge_join/classic_join.rs	87.45%	8 Missing and 63 partials ⚠️
...ysical-plan/src/joins/piecewise_merge_join/exec.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #23870    +/-   ##
========================================
  Coverage   80.71%   80.72%            
========================================
  Files        1090     1090            
  Lines      370339   370906   +567     
  Branches   370339   370906   +567     
========================================
+ Hits       298925   299411   +486     
- Misses      53605    53607     +2     
- Partials    17809    17888    +79     

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