Summary

This PR was created with the help of AI, albeit with heavy oversight and review.

This is part of a series of PRs implementing the vector graph type for the Interactive Graph widget:

PR1 – type definitions and schema
▶️ PR2 – state management (this PR)
PR3 – rendering & accessibility
PR4 – scoring
PR5 – editor support

Issue: LEMS-3971

• Added state management logic for the vector graph type
• The graph is now stateful and interactive but not yet visible — rendering still throws "Not implemented" from PR 1's stub. This is a pure additive change — no runtime behavior is altered and all existing tests continue to pass.

Changes:

• interactive-graph-action.ts — Adds vector: { moveTip, moveVector } to the actions object. moveTip dispatches movePoint(1, destination) for the tip (the only draggable point). moveVector dispatches moveLine(0, delta) for body translation.
• initialize-graph-state.ts — Replaces the PR 1 stub with a real case "vector". Adds and exports getVectorCoords() with the standard three-path fallback: explicit coords → startCoords → normalized defaults. Default is a horizontal vector from [-5, 0] to [5, 0].
• interactive-graph-reducer.ts — Adds vector case in doMovePoint (snap to grid, enforce bounds, reject if tip would overlap with tail). Adds "vector" to the existing doMoveLine case alongside "linear" / "ray" for body translation.
• interactive-graph-state.ts — Adds getGradableGraph() branch for vector (coords passthrough, same shape as ray).
• interactive-graph-question-builder.ts — Adds withVector() builder method and VectorGraphConfig class (matches RayGraphConfig pattern).
• interactive-graph-widget-generator.ts — Adds generateIGVectorGraph() generator function (new pattern, replacing builder long-term).
• perseus-core/src/index.ts — Re-exports generateIGVectorGraph.
• perseus/src/index.ts — Re-exports getVectorCoords.
• interactive-graph.testdata.ts — Adds vectorQuestion and vectorQuestionWithDefaultCorrect test fixtures.

Test plan:

• pnpm tsc — no new type errors
• pnpm test packages/perseus/src/widgets/interactive-graphs/reducer/ — 143 tests pass, including 9 new vector tests:
  • 3 initialization tests (explicit coords, startCoords fallback, defaults)
  • 3 moveTip tests (basic move, hasBeenInteractedWith, overlap rejection)
  • 3 moveVector tests (body translation, hasBeenInteractedWith, bounds enforcement)
• No existing interactive graph tests regress