fix(gfql): support cuDF index byte reporting

[lmeyerov]

Summary

• Make show_indexes() byte accounting engine-aware for pandas, Polars, and cuDF series.
• Treat failed GPU detection as a test skip rather than an error.
• Preserve compatibility with the repository's engine-agnostic SeriesT type-hygiene policy.

Validation

• Type-hygiene guard and focused Ruff: PASS.
• CPU/Polars focused index suites (test_node_type_segments.py + test_index.py, excluding cudf/cupy): 291 passed, 93 deselected, 2 xfailed.
• Exact combined release candidate 0319ae230...: local pandas/Polars MCP acceptance PASS and guarded DGX cuDF 26.02 MCP acceptance PASS.

Evidence

• Found by the release MCP harness: cuDF show_indexes() raised Series has no attribute nbytes on master f4029d862....
• Exact head: 79a5d1b6dc57dce527f5785c652c8ce6a7442ee9; tree: 29781691c73aee5844c726739f9779d3432f8a45.
• CI Tests run: 39043825831: SUCCESS; CodeQL run 39043825813: SUCCESS; RTD: SUCCESS.

Notes

• Release-only and human-gated; this PR does not authorize release, tagging, deployment, or release-PR merge.