feat: merge schemas as numpy promotes types

[robinskil]

Summary

The default strategy refuses a boolean beside a number. A collection from numpy or xarray expects the rules of numpy.result_type.

This PR adds NumpyArrowTypeWidening, a second ArrowTypeWideningStrategy. It applies the numpy rules:

• A boolean joins the numbers.
• Float16 joins the floats.
• A narrow integer beside a Float32 stays a Float32.
• A number beside a string reads as text.
• A date beside a timestamp is a timestamp at the finer unit.

numpy resolves a set of types at once. The strategy gathers the types of each column across every schema and resolves the set once. The file order does not change the result.

Configuration

Set BEACON_TYPE_WIDENING_STRATEGY=numpy on the server. Pass the strategy to RuntimeBuilder::with_type_widening in an embedded build. BEACON_TYPE_WIDENING_ON_CONFLICT applies under both strategies.

Limits

Arrow has no cast for four numpy rules. These pairs stay conflicts: integer beside duration, duration beside timestamp, number or text beside binary. The CSV reader parses text as the merged type. A boolean literal beside a number fails at read time.

Tests

The tests embed the numpy.promote_types table from numpy 2.5.2. They check every Arrow pair against it and the Arrow cast of every promotion.

[codecov]

Codecov Report

❌ Patch coverage is 98.47434% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.41%. Comparing base (294e328) to head (bd596a6).

Files with missing lines	Patch %	Lines
...b/beacon-datafusion-ext/src/type_widening/numpy.rs	99.10%	6 Missing ⚠️
beacon-db/beacon-core/src/embedded.rs	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #478      +/-   ##
==========================================
+ Coverage   83.23%   83.41%   +0.18%     
==========================================
  Files         373      374       +1     
  Lines       62526    63237     +711     
==========================================
+ Hits        52044    52750     +706     
- Misses      10482    10487       +5     

Files with missing lines	Coverage Δ
beacon-db/beacon-core/src/runtime_builder.rs	92.23% <ø> (ø)
...acon-db/beacon-datafusion-ext/src/type_widening.rs	98.69% <100.00%> (+0.22%)	⬆️
beacon-server/beacon-server-config/src/lib.rs	95.77% <100.00%> (+0.99%)	⬆️
beacon-server/beacon-server/src/server/mod.rs	84.48% <100.00%> (ø)
beacon-db/beacon-core/src/embedded.rs	68.46% <28.57%> (-0.69%)	⬇️
...b/beacon-datafusion-ext/src/type_widening/numpy.rs	99.10% <99.10%> (ø)

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