Problem

Source grid/flex containers retain classes but are serialized as Group blocks with is-layout-flow and wp-block-group-is-layout-flow. Core child margins/gaps then coexist with authored CSS, while generated paragraph/group wrappers alter direct-child topology. Fixture 37 amplifies this in .ex-row, nested Contact/Visit sections, header chrome, and other CSS-owned grids.

Work

• Detect containers whose resolved author CSS owns grid/flex/spacing geometry.
• Preserve source semantic tag and direct-child topology where representable.
• Prevent core flow gap/margin defaults from contributing inside author-owned layouts.
• Emit diagnostics when Gutenberg save shape forces topology changes that invalidate structural selectors.

Acceptance

• A five-cell authored grid retains child order/count and computed placement without injected flow spacing.
• Nested source sections retain semantic tags and authored padding.
• Ordinary Group flow layouts retain core defaults.
• Fixture 37 exhibition rows and Contact/Visit section geometry improve without fixture-specific CSS.

Evidence: Lab run d17780c9-0100-4390-abe0-43eb11d465e7; source .ex-row, .form, and global section rules. AI-assisted diagnosis: GPT-5.6 Sol via OpenCode; Chris owns acceptance.