test: isolate legacy cross-layer unit fixtures

Isolate deliberately cross-layer legacy unit fixtures.
Keep architecture rules honest without losing regression coverage.

Full context for agent readers

## Problem/Goal

`src/platform/settings/composables/useLitegraphSettings.reactiveEdge.test.ts` deliberately imports the renderer-owned canvas store to reproduce a legacy synchronization edge. Because the fixture lives under `src/platform/**`, it needs an `import-x/no-restricted-paths` suppression even though the production layering rule is correct.

Create an explicit higher-level home for unit tests that must exercise legacy cross-layer behavior, analogous to `browser_tests/tests/legacyDoNotReplicate`, so exceptional fixtures do not weaken or locally suppress production-layer restrictions.

Origin: https://github.com/Comfy-Org/ComfyUI_frontend/pull/15980#discussion_r3868366096

## Proposed Solution

- Add a clearly named top-level unit-test fixture directory outside the production layer targets, using repository-consistent naming such as `legacyDoNotReplicate`.
- Move the reactive-edge regression fixture there without changing its behavior.
- Ensure Vitest, coverage, lint, and typecheck include the new location intentionally.
- Remove the fixture's `import-x/no-restricted-paths` suppression; do not relax the production layer zones.

## Acceptance Criteria

- The `CanvasInfo` explicit-source regression remains green at the fixed implementation and fails under the prior implicit `watchEffect` behavior.
- The lint detector and its existing semantic regression coverage remain unchanged and green.
- No production layer restriction is disabled, widened, or bypassed.
- The moved fixture requires no restricted-path suppression.
- The directory name and nearby documentation make clear that its cross-layer patterns are legacy-only and must not be copied into production code.

Start with src/platform/settings/composables/useLitegraphSettings.reactiveEdge.test.ts and trace how Vitest, coverage, lint, and typecheck discover unit-test fixtures. Move the regression fixture to a clearly named top-level legacy-only directory, preserve its behavior, and remove the restricted-path suppression. Done means the explicit-source regression and lint detector coverage stay green without weakening production layer rules.

Written by the indexing model from the issue text.