feat(tables): add first-class fixed-size-list GPU columns

[ibgreen-openai]

Goals

Add first-class fixed-size GPU table columns such as fixed-size-list<float32,768> without introducing a second embedding-specific storage owner.

Changes

• Add the public FixedSizeList<Format, Size> type and canonical fixed-size-list format decoding.
• Preserve logical row counts, flattened coordinate counts, padded rows, existing chunk/batch boundaries, and GPUData ownership.
• Keep fixed-size-list storage distinct from vertex attributes and validate actual storage-binding spans.
• Document GPU table format, schema, vector, data, lifecycle, and explicit packing semantics.
• Cover widths 384/768/1536, empty rows, padding, ownership, explicit attribute expansion, and safe nullable packing.

Previously reviewed as #3007. This is the clean landing PR based on the graph prerequisite; merge only after #2943 lands in master.

[chatgpt-codex-connector]

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