Isolate the PDF interpolation backend to support both LHAPDF and NeoPDF

.github/workflows/rust.yml

@@ -35,6 +35,7 @@ jobs:
           # `-C link-dead-code` is needed to prevent 'warning: XX functions have mismatched data' warnings
           RUSTFLAGS: '-Cinstrument-coverage -Clink-dead-code'
         run: |
+          export NEOPDF_DATA_PATH=/usr/local/share/LHAPDF
           # we need stderr, but we can't run test twice because it'll regenerate/modify the binaries which interferes with `llvm-cov`
           cargo test --features=applgrid,evolve,fastnlo,fktable --no-fail-fast 2> >(tee stderr 1>&2)
           # from https://stackoverflow.com/a/51141872/812178

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Isolate PDF interpolation backend, current suppored backends include
+  LHAPDF and NeoPDF
+
 ## [1.4.0] - 12/05/2026
 
 Starting with this version, PineAPPL has an official logo!