[test] Add a unit-test baseline for the canvas and rules layers #172

Context

The test suite (~224 files) is mostly backend rule tests, while tests/browser/ contains only a few "verify" scripts. The canvas component (src/canvas/CanvasWorkspace.tsx, 6,761 lines) and the frontend rules module (src/backend/rules.ts, 631 lines) are mainly covered end-to-end, with little or no unit coverage. This makes refactoring those areas high-risk.

Scope

• Add unit tests for the canvas interaction surface: selection, dragging, connections, generation submission, history, and import/export.
• Add unit tests for src/backend/rules.ts and its derived/projection logic.
• Keep tests deterministic and behaviour-focused (avoid asserting on implementation details).
• Wire the new tests into CI.

Acceptance criteria

• Key canvas interactions and rule/derivation functions have repeatable unit tests.
• The new tests run in CI and fail on regressions.
• No product behaviour changes.

Dependencies / notes

• Safety net and prerequisite for the CanvasWorkspace split and the frontend-rules deduplication issues.
• Static review baseline: commit a1ff887.