feat(holder): add configurable holder-edge spacing

[richardsolomou]

Problem

Holder slot spacing did not control the clearance between slot recesses and the holder edge, which could leave slots too close to the perimeter or require more module space than expected.

Changes

• Add configurable holder-edge spacing with a default of half the miniature spacing.
• Preserve the default relationship when miniature spacing changes until edge spacing is customized.
• Account for edge spacing during holder packing, capacity limits, and module sizing.
• Migrate saved workspaces to the new setting and document the behavior in the README.
• Add unit and end-to-end coverage for defaults, customization, packing, and migration.

Risk

Low. Holder layouts and persisted workspace loading are affected; the change can be rolled back with the existing commit or release rollback.

Verification

• Added holder geometry unit tests for defaults, edge clearance, and additional column selection.
• Added workspace migration coverage for legacy saved settings.
• Added end-to-end coverage for default, customized, and reset edge spacing behavior.

Checklist

• Tests cover new or changed behavior where practical.
• Geometry changes are checked against a real export, not just the preview.
• User-facing behavior is documented in the README.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	basekit	7370713	Commit Preview URL Branch Preview URL	Aug 31 2026, 12:53 PM