chore: native ANN vector index on the Tensors stack; drop external FaissNet

[ooples]

Native ANN vector index on the AiDotNet stack — replaces external FaissNet

Consumes the native fused ANN kernels from AiDotNet.Tensors #824 to give FAISS-style IVF/PQ/IVFPQ/Flat search with no external native dependency, and removes the broken FaissNet backend.

Why: FaissNet's IVF/PQ path is unusable — it ships an incomplete MKL redist and dies with a process-fatal Intel MKL FATAL ERROR: Cannot load mkl_def.2.dll that can't be caught. This PR moves the whole ANN stack onto AiDotNet's own Tensors kernels.

What's here

• AnnVectorIndex<T> : IVectorIndex<T> — Flat / IVF / PQ / IVFPQ over Tensors' AnnIndex. Bridges the incremental Add/Remove/Search contract onto a train-then-populate ANN index (live-vector source of truth + lazy rebuild), and AttachGpu(...) dispatches distance / assignment / PQ scans to the fused GPU ANN kernels (IAnnBackend) across all 7 backends, falling back to the managed CPU reference. Reuses the existing VectorIndexDocumentStore adapter, so it plugs into the RAG facade unchanged.
• ConfigureNativeAnnIndex(...) on both IAiModelBuilder and the concrete builder — surfaces the native index from the facade with optional GPU acceleration.
• Bump AiDotNet.Tensors 0.114.0 → 0.118.0 (the gating dependency: Tensors fix: allow setparameters on untrained time series models #824's native ANN kernel stack).
• Removes the AiDotNet.Storage.Faiss project, its tests, the FaissNet package reference, and all solution/csproj wiring — the native path supersedes it.

Verification

• Core builds clean against Tensors 0.118.0.
• AnnVectorIndexTests 9/9 green on net471 + net10: exact flat NN, in-cluster recall for IVF/PQ/IVFPQ on separable clusters, Add/Remove/Clear/AddBatch, cosine normalization.

Merge order

Base is feat/rag-productionization (#1896) since the FaissNet code being removed was added there. Requires publishing AiDotNet.Tensors 0.118.0 (from Tensors PR #824) before CI restore goes green; verified locally against a 0.118.0 package built from that branch.

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Original title:
Native ANN vector index on the Tensors stack; drop external FaissNet

New title:
chore: native ANN vector index on the Tensors stack; drop external FaissNet

Detected type: chore: (default type)
Version impact: No release

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