feat(atoms): replace arrays instead of deep merging in mutate shorthand

[bowheart]

Summary

• Arrays in .mutate shorthand now fully replace instead of deep merging by index. The previous behavior was surprising — e.g. signal.mutate({ arr: [1, 2] }) on state { arr: [10, 20, 30] } produced { arr: [1, 2, 30] }. Now it produces { arr: [1, 2] }. The callback form still supports index-level updates: signal.mutate(s => { s.arr[2] = 30 })
• Fixes pre-existing type errors in AtomInstance, AtomTemplate, AtomTemplateBase, atom.ts, and ion.ts by loosening the AtomApiGenerics Signal constraint from bare Signal to AnySignal (Signal<any>)
• Consolidates RecursivePartialWithArrayPlucking into RecursivePartial — adds an array guard to RecursivePartial in both @zedux/core and @zedux/atoms so arrays are treated as leaf values instead of having their methods made optional

This reverts the array pluck shorthand introduced in v2.0.0-rc.13 from #331 6 days ago, so I'm not too concerned about it breaking stuff. Now that .mutate shorthands can overwrite values with different value types (e.g. objects replacing arrays and vice versa), the pluck shorthand is strange. It's better to use normal mutation in the callback form for updating individual array elements anyway.

Test plan

• All 45 proxy integration tests pass with updated expectations
• tsc --noEmit passes with zero errors for the atoms package
• Verify no regressions in downstream consumer code that relied on array deep merge behavior

🤖 Generated with Claude Code