#1817 sycl: use the source version for setup model checks

Beschreibung

## Summary

Fixes #1794. Use `setup.source_version()` for the SYCL installer's stub `BUILD.json`. Its old parser expected a literal version in the SYCL `project()` declaration, which now uses `${STRATA_SYCL_VERSION}`; it returned `0` and failed the Unsloth model version checks.

This follows the top-level project version without changing model minimums, build flags, or CUDA/HIP setup. Thanks to agnaylor for the reproduction.

## Test plan

- [x] New tests fail before the fix and pass afterward: the current source version satisfies both model minimums, and a four-part source hotfix is retained despite the variable SYCL declaration.
- [x] `tools.test_setup_sycl`, `tools.test_setup_choices`, `tools.test_setup_pins`: 74 tests pass on Linux, Python 3.14.7.
- [x] Python syntax check.
- Not tested: an Intel GPU build, full model setup, or inference. No engine/build sources changed.

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