feat(react): lift array add/remove state out of the replaceable root (#145)

[timkindberg]

Summary

Closes #145 (child of #141). Stacks on #150. Related to #111.

defaults.array.root can arrange like a group (Label + {children} + AddButton) without wrapping <Default />. Add/remove state lives in a provider around the template, so replacing root doesn't kill Add. Item chrome is defaults.arrayItem.

Engine children are still just the seed items; the provider passes the live slot list as children. No Core change.

Agent did the grind.

Test plan

• Default arrays still add/remove, dense re-path, keep typed values
• Custom array.root that places Label + children + AddButton still has working Add/Remove
• Replacing defaults.array.root does not lose Add
• CI green

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