Configure horizontal screws per edge

[yawkat]

Summary

• replace the single horizontal screw toggle with north, east, south, and west booleans
• allow horizontal holes to cut directly into the grid when no plate wall is present
• restrict holes to outer plate edges instead of internal segment connector edges
• update the web editor configuration and README example

Validation

• just test
• editor.toml schema and parameter metadata validation
• manifold wall-mounted example render with hard warnings
• manifold four-segment render confirming holes only on the outer perimeter
• git diff --check

[github-actions]

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