Add FixedPointDecimals package extension for FixedDecimal → Dec128 BSON representation

[Copilot]

FixedDecimal types from FixedPointDecimals.jl had no BSON representation, requiring manual conversion. This adds a weak dependency extension that maps FixedDecimal{T,f} to BSON decimal128 via Dec128, using precise decimal arithmetic throughout.

Changes

• ext/LightBSONFixedPointDecimalsExt.jl — new package extension (loaded only when FixedPointDecimals.jl is present):

  • bson_representation_type(FixedDecimal{T,f}) = Dec128
  • Forward: Dec128(x.i, -f) — exact conversion using raw integer coefficient and scale
  • Reverse: reinterpret(FixedDecimal{T,f}, T(x * Dec128(1, f))) — reconstructs the raw integer via decimal-only arithmetic, avoiding binary float precision loss

• Project.toml — adds FixedPointDecimals as a weak dependency with compat "0.5, 0.6", registers the extension, bumps julia compat to "1.9" (minimum for package extensions), and adds FixedPointDecimals to test extras/targets

• test/fixed_decimal_tests.jl — roundtrip tests for multiple scale factors; confirms conversion is bit-exact

Usage

using LightBSON, FixedPointDecimals, DecFP

x = FixedDecimal{Int64, 2}(1.23)  # internal: i=123, f=2

buf = UInt8[]
writer = BSONWriter(buf)
writer["x"] = x          # stored as Dec128("1.23") via Dec128(123, -2)
close(writer)

BSONReader(buf)["x"][FixedDecimal{Int64, 2}]  # == 1.23, exact roundtrip

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.12%. Comparing base (6173a7b) to head (e700c7b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   90.08%   90.12%   +0.03%     
==========================================
  Files          13       14       +1     
  Lines         787      790       +3     
==========================================
+ Hits          709      712       +3     
  Misses         78       78              

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