Phase 2 of the consumer side (follows #114): let third-party plugins render host-owned datasets without ever receiving a copy of the rows.

• DataView render node — { handle, view }. A plugin emits it inside its own UI tree; the host, while drawing the tree, replaces it with the data read from its single owned copy (datasets::read). Rows never enter the plugin's sandbox. Additive to the RenderNode set; #[serde(other)] Unknown means older peers ignore it (no breakage).
• data-consumer capability + host dataset-bus import — a plugin declares it consumes datasets, requests one via the bus (routed through the user's producer pick), gets a handle, and can read(handle, offset, limit) paged when it genuinely needs raw values.
• Plugin renderers as view options. A consumer plugin that can render a dataset appears as an additional format in the Dataset Viewer (Host Dataset Viewer — single copy + notify/pull freshness + format switcher #114), alongside the built-in table / json-tree / chart.

Depends on #114 (registry + viewer + notify/pull) and #116 (consent for cross-plugin access).

Releasable: a third-party plugin renders another tab's data host-side by handle, with no duplicate copy, chosen by the user as a view format.

Part of the plugin data ecosystem epic #118.