PAR-004: xnec2c-style workbench parity acceptance criteria (Phase 3)

[dc0sk]

Source: docs/backlog.md (PAR-004)

Owner: GUI+CLI
Target: Phase 3

Resolution criteria:

• usability acceptance checklist defined and demonstrated
• includes interactive sweep inspection, graphical result browsing, and fast edit-run-inspect loop
• at least one end-to-end demo captured

Notes:
Tracks PRT-004 / CP-005 alignment.