Rename rendered → canvas across the graph path and add Canvas terminology

## Task

Rename `rendered` → `canvas` across the graph render path, and establish "Canvas" as the ubiquitous term for on-graph entities in `CONTEXT.md`. Pure rename plus terminology — no behavior change.

## Details

- Branded types: `RenderedVertexId` → `CanvasVertexId`, `RenderedEdgeId` → `CanvasEdgeId`.
- Rename the converter module (`renderedEntities` → `canvasEntities`) and its test.
- Rename the id helpers, converter functions, hooks, and element types (`RenderedVertex`/`RenderedEdge` → `Canvas*`) and all call sites (~11 files, including the Graph View, context menu, node badges, badge renderer, global actions, Schema View sidebar, state-provider index).
- **Collision check (done):** target names (`CanvasVertexId`, `CanvasEdgeId`, `toCanvasVertex/Edge`, `useCanvasVertices/Edges`, `canvasEntities`) do not currently exist. Existing `canvas` usage is the unrelated HTML/cytoscape-canvas sense (`CytoscapeCanvas`, `cyCanvas`, `getCanvas`) — no conflict.
- **CONTEXT.md:** add a **Canvas Vertex** / **Canvas Edge** term (the structural, id-prefixed representation Cytoscape renders) and record the `rendered` → `canvas` rename under Flagged ambiguities, consistent with the other legacy-term migrations.

## Acceptance

- No `rendered`/`Rendered*` identifiers remain in the graph path (verified by grep).
- `pnpm check:types` and `pnpm test` green.
- `CONTEXT.md` carries the Canvas terminology.

## Related Issues

- Parent #1887

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.

Start with the renderedEntities converter module and its test, then trace the graph path call sites listed in the issue, including Graph View, context menu, node badges, badge renderer, global actions, Schema View sidebar, and the state-provider index. Verify the rename with grep, run pnpm check:types and pnpm test, and confirm CONTEXT.md contains the Canvas Vertex and Canvas Edge terminology.

Written by the indexing model from the issue text.