[Fix] Consistent promotion rules for Vector and Numeric

[sjfeng1999]

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Technical Details

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[Copilot]

Pull request overview

This PR updates FlyDSL’s dtype promotion so Vector operators and scalar Numeric operators share consistent promotion and result-type rules, and strengthens unit tests to catch vector/scalar drift and a specific mixed-width integer regression.

Changes:

• Refactors promotion into shared helpers (_coerce_operands, _result_type_for_op) and applies them to Vector arithmetic.
• Updates Vector unit tests to expect auto-promotion (no explicit .to() required) and adds an Int32x4 + Int64x4 regression check.
• Expands numeric promotion tests to assert vector/scalar dtype consistency across more operators.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/unit/test_vector.py	Updates expectations to vector auto-promotion and adds a mixed-width integer vector regression test.
tests/unit/test_numeric_promotion.py	Refactors _binop to exercise both scalar and vector ops and asserts dtype consistency across more operators.
python/flydsl/expr/typing.py	Moves Vector operator dtype resolution to the shared scalar promotion/result-type helpers.
python/flydsl/expr/numeric.py	Introduces shared promotion/result-type helpers and adjusts operator implementation accordingly.

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[sjfeng1999]

rebase #789 later