Accept backend source-map byte spans in CrossTL reports

[NripeshN]

Summary

• accept generated backend byte/column spans in CrossTL backend source-map sidecars
• validate backend byte span fields as an all-or-none group with endOffset consistency
• update project-report backend source-map fixture coverage for byte spans and malformed byte-span cases

Validation

• cmake --build build --target cglc crossgl_unit_tests -- -j12
• ctest --test-dir build --output-on-failure -R 'cglc_(crosstl_project_report_backend_source_map|cli_check_crosstl_project_report_backend_source_map|cli_dump_backend_source_map|dump_backend_source_map_)' -j 12
• python3 tools/check_invalid_json_schema_fixtures.py --root . --jobs 12
• ctest --test-dir build --output-on-failure -E '^crossgl_unit_tests$' -j 12
• pre-commit run --all-files

Note: crossgl_unit_tests binary builds locally, but the test executable currently fails native Metal metallib expectations because this shell has no discoverable metallib tool (CMake reports metal=FALSE / missing CROSSGL_METALLIB).