fix(Table): fix grouped header alignment and duplicate key warnings & add regression tests

[seanrcollings]

Resolves #267

[Copilot]

🔵 Needs a closer look

Placeholder keys can still collide, and the playground toggle does not reproduce the reported nested-header case.

Review details

Suppressed comments (1)

packages/atomic-elements/src/components/Content/Table/components/TableHeader.tsx:50

• node.index is only assigned explicitly to real leaf clones in TableCollection; the synthetic placeholders created by buildHeaderRows are positioned by colIndex, and their index can be absent. That makes multiple placeholders in one row receive the same ...-undefined id, so the duplicate-key warning and stale-cell bug can remain. Use the placeholder's colIndex (or the callback's actual sibling index) for this position-based id.

          id={`${row.key}-placeholder-${node.index}`}

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

All reviewed changes are covered and no unresolved blocking issues remain.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite