Keep separator geometry block-valid

[chubes4]

Summary

• retain only top/bottom margins supported by core/separator in canonical block attributes
• move source horizontal auto margins and max-width into the generated geometry carrier stylesheet
• preserve exact rendering while matching Gutenberg save output

Evidence

• composer test:canonical passes on current trunk
• Fixture 37 front-page editor validity improved from 183/184 to 184/184
• all seven editor-validated surfaces report zero invalid blocks
• eight captured surfaces retain zero visual mismatches
• Before: Homeboy run a77629e4-0ec3-4813-90c4-087f305c47e2; after: 5aedff96-df46-466e-b2db-e7e84e88b347

AI assistance

• Model: OpenAI GPT-5.6 Sol
• Tool: OpenCode
• Use: editor-failure diagnosis, implementation, regression coverage, canonical testing, and rendered/editor evidence analysis

Chris Huber reviewed and remains responsible for the change.