feat(ui): show variant stock indicator in parts table

[br413]

Summary

• Add variant and direct stock breakdown to the parts table hover card when variant_stock > 0
• Extract shared renderPartStockCell renderer and reuse it in Parametric View for consistency
• Matches the existing build allocation / BOM pattern using TableHoverCard

Fixes #5959

Test plan

• Create a template part with variants that have stock
• Confirm parts table shows info icon next to stock count
• Hover shows "Includes variant stock" and "Direct stock" breakdown
• Confirm Parametric View shows the same hover behavior
• Confirm parts without variant stock are unchanged

[netlify]

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