obs4REF FLUXNET2015 metadata inconsistency blocks dict-form migration of gpp-FLUXNET2015

## Problem

`gpp-FLUXNET2015` is the one ILAMB diagnostic whose reference data lives in obs4REF but which cannot be moved to a dict-form `sources` entry (the form that makes the reference visible to the solver and the solve-regression tests). It is stuck as a string-form registry-key source in `packages/climate-ref-ilamb/src/climate_ref_ilamb/configure/ilamb.yaml`.

## Root cause

The dataset spells its `source_id` three different ways: `FLUXNET2015-1-0` in the obs4REF path, `Fluxnet-2015-1-0` in the filename, and `Fluxnet-2015` as the internal netCDF attribute. This will break if published as is

## Fix

Re-mint / rename the obs4REF FLUXNET2015 dataset so path, filename, and internal `source_id` attribute all agree on one spelling.

## Impact until fixed

`gpp-FLUXNET2015` still executes correctly (reference resolved at execute-time via the registry key), but its solve-regression snapshot shows only CMIP model data, not the obs4REF reference.