fix: append scalars when Set targets array index beyond length

[Solaris-star]

Summary

Set with successive array indexes on a newly created path overwrote the whole array instead of appending:

Set({}, 1, "test", "[0]") // {"test":[1]}
Set(..., 2, "test", "[1]") // was {"test":[2]}, want {"test":[1,2]}

Root cause: when extending an existing non-empty array, only arrays whose next token was { were treated as appendable. Scalar arrays took the overwrite branch.

Fix

Treat any non-empty array as extendable when the next key is an index ([n]), matching object-append behavior and issue #267.

Test plan

• go test -run TestSet .
• TestSetAppendsScalarToExistingArrayByIndex
• updated supplemental coverage expectation to append (not overwrite)

Fixes #267