Reduce executor allocations and fix scalar/outer-join correctness

[KKould]

Summary

• Fix scalar cardinality validation, hash outer-join match tracking, and TopK ordering.
• Reduce unnecessary tuple copies and allocations; preallocate executor nodes and reuse CSV/TopK buffers.
• TupleBuilder::build_with_row now fills a caller-owned Tuple.

Tests

• Workspace tests with spill; lib tests with copy,lmdb,decimal,spill.
• All 196 SQLLogicTest files; formatting and diff checks.

Performance gains have not been benchmarked.

[codecov]

Codecov Report

❌ Patch coverage is 98.94459% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.30%. Comparing base (d980efa) to head (4dcd111).

Files with missing lines	Patch %	Lines
src/execution/dql/scalar_apply.rs	92.10%	3 Missing ⚠️
src/execution/dql/join/nested_loop_join.rs	97.91%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
+ Coverage   93.26%   93.30%   +0.03%     
==========================================
  Files         259      259              
  Lines       47741    47765      +24     
==========================================
+ Hits        44524    44565      +41     
+ Misses       3217     3200      -17     

Flag	Coverage Δ
rust	93.30% <98.94%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/catalog/table.rs	98.79% <100.00%> (+0.01%)	⬆️
src/db.rs	92.04% <100.00%> (ø)
src/db/prepared.rs	94.55% <100.00%> (ø)
src/execution/ddl/add_column.rs	96.93% <100.00%> (-0.04%)	⬇️
src/execution/ddl/change_column.rs	97.08% <100.00%> (ø)
src/execution/ddl/create_index.rs	91.25% <100.00%> (+0.22%)	⬆️
src/execution/dml/analyze.rs	98.00% <100.00%> (-0.01%)	⬇️
src/execution/dml/copy_from_file.rs	97.50% <100.00%> (+0.02%)	⬆️
src/execution/dml/copy_to_file.rs	99.09% <100.00%> (+0.80%)	⬆️
src/execution/dml/delete.rs	100.00% <100.00%> (ø)
... and 22 more

... and 2 files with indirect coverage changes

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