docs(research): CUDA 13.3 impact assessment for VMAFX fork

[lusoris]

Summary

• Adds docs/research/research-0734-cuda-13.3-impact-assessment.md — a scoping digest for the CUDA 13.3 toolkit bump, covering Tile C++ kernel rewrites, CompileIQ autotuning, green contexts for vmafx-server, DLPack interop in the ONNX Runtime path, and NVCR/Jimver image availability.
• Per-item recommendation table: wait-for-benchmark (Tile C++), do-now pilot (CompileIQ on filter1d.cu), wait-for-Torch-TRT (green contexts), not-worth-it yet (DLPack), wait-for-NVCR (image availability).
• Includes LOC + complexity estimates per CUDA kernel candidate measured against the live core/src/feature/cuda/ tree (3 049 LOC across the candidate set; 845-LOC filter1d.cu is the strongest Tile-C++ structural match).
• All cited claims survived a 3-vote adversarial verification pass; refuted claims (e.g., the "Tile C++ is performance-portable without per-arch tuning" marketing framing, several DLPack-in-ORT historical claims) are deliberately omitted.

Test plan

• Reviewer: confirm per-kernel LOC numbers against wc -l core/src/feature/cuda/**/*.cu
• Reviewer: confirm grep -rn 'DLPack\|dlpack' core/src/dnn/ returns no hits (digest's "current state" baseline)
• No code changes; doc-only PR; no build / test gates apply beyond markdown linting

Deliverables (ADR-0108)

• research digest: this PR is the digest
• decision matrix: per-item recommendation table in §"Per-item recommendations"
• no AGENTS.md invariant note needed: no rebase-sensitive invariants changed
• reproducer / smoke command: wc -l core/src/feature/cuda/**/*.cu and grep -rn 'DLPack' core/src/dnn/
• no changelog fragment needed: docs-research-only
• no rebase impact: docs-research-only

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