test: dfd-node-properties stroke-color test mismatches color-picker palette gating

[ericfitz]

Summary

e2e/tests/field-coverage/dfd-node-properties.spec.ts ("stroke color via style panel") times out waiting for the color-picker hex input. The input is gated by src/app/shared/components/color-picker/color-picker.component.ts:88-91 on selectedSource === 'diagram' && selectedDiagramPosition !== null (diagram-palette editing mode), not simply on a node being selected. The test predates this gating design and never enters palette mode.

Confirmed unrelated to the x6 v3 migration (#446): the style-panel data chain (buildStylePanelCells()) is null-safe and populates correctly; the component is untouched by the migration.

Acceptance Criteria

• Decide intent: should the hex input be reachable with just a node selected (UI fix), or must the test drive palette mode first (test fix)?
• Update test or component accordingly; test passes in the full field-coverage run.

Notes

Root-caused live during #446 E2E validation (2026-08-03) via ng.getComponent() inspection; details in the #446 execution report.