v1.2.5: parameter editor with auto-load from API + params-render.js
Phase 1: paramDefsCache, step.params as {name:value}
Phase 2: resolveStepSvcOpId() → loadStepParams() via /api/params
Phase 3: <details> in step card, renderParamRow with pre-fill (clone def + saved values)
Phase 4: snapshotStepParams() before re-render, reset on operation change
Phase 5: collectFormSteps via collectParams() + numeric→name reverse mapping
Only scenario-form.js changed. params-render.js, api_test.py, _resolve_params unchanged.
Compatible with existing scenarios (symbolic names in DB).