feat: support common table expressions

[KKould]

Summary

• add non-recursive CTE binding with nested scope handling
• add spill-backed WITH RECURSIVE ... UNION ALL execution
• reuse the recursive execution arena between generations
• integrate recursive operators with planning, optimization, and EXPLAIN
• add SQLLogicTest and executor-level spill/generation coverage

Validation

• cargo fmt --all -- --check
• cargo test --lib --features spill,decimal iteration_reads_only_the_previous_spilled_generation
• cargo clippy --features spill --lib
• make test-slt SQLLOGIC_PATH=tests/slt/cte.slt

[codecov]

Codecov Report

❌ Patch coverage is 96.33943% with 22 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@bccf67f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/binder/parser.rs	96.07%	6 Missing ⚠️
src/execution/dql/recursive_cte.rs	97.40%	6 Missing ⚠️
src/execution/spill/mod.rs	78.57%	6 Missing ⚠️
src/planner/operator/recursive_cte.rs	87.50%	2 Missing ⚠️
src/binder/select.rs	94.73%	1 Missing ⚠️
src/optimizer/rule/normalization/column_pruning.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #379   +/-   ##
=======================================
  Coverage        ?   92.95%           
=======================================
  Files           ?      255           
  Lines           ?    43770           
  Branches        ?        0           
=======================================
  Hits            ?    40687           
  Misses          ?     3083           
  Partials        ?        0           

Flag	Coverage Δ
rust	92.95% <96.33%> (?)

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

Files with missing lines	Coverage Δ
src/binder/expr.rs	94.26% <100.00%> (ø)
src/binder/mod.rs	86.69% <100.00%> (ø)
src/execution/dql/mod.rs	100.00% <ø> (ø)
src/execution/mod.rs	95.12% <100.00%> (ø)
src/optimizer/rule/implementation/mod.rs	92.49% <100.00%> (ø)
...mizer/rule/normalization/compilation_in_advance.rs	96.00% <100.00%> (ø)
src/optimizer/rule/normalization/mod.rs	83.67% <100.00%> (ø)
src/planner/arena.rs	99.69% <100.00%> (ø)
src/planner/mod.rs	93.75% <100.00%> (ø)
src/planner/operator/mod.rs	96.40% <100.00%> (ø)
... and 8 more

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