Harden generated Show view configurations

[DevGiuDev]

Summary

• add shared guidance for valid Show layout segment and column pairs
• tell view-generation agents to use blank for literal labels and never emit type: "text"
• require existing and plugin-specific configuration to be preserved during updates
• render generated configuration in a native <details> disclosure, closed by default
• escape JSON values and constrain expanded previews with scrolling

Validation

PASS tests/view-layout-guidance.test.js
PASS tests/generated-view-preview.test.js
6 tests passed

Closes #53