Add failing test for Seq/Array atOrElse IndexOutOfBoundsException (issue #301)

[adamw]

Addresses #301.

The atOrElse operator on Seq/Array currently throws IndexOutOfBoundsException when accessing a missing index instead of using the provided default value. This test demonstrates the expected behavior.

The test includes two cases: modifying an existing element at a valid index (current working behavior), and accessing a missing index with atOrElse without throwing (expected behavior that currently fails).

The actual fix implementation is pending.

Closes #301.

[adamw]

The CI failure is a compilation error, not a test failure. The atOrElse method does not exist on List — the test file references it, but the implementation hasn't been added yet.

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CI failure summary:

Compilation fails across all Scala 2.x targets on ModifySeqAtOrElseTest.scala. The test calls _.atOrElse(index, default) on List, but the method doesn't exist in the library yet — the PR adds only the failing test, not the implementation.

Specific errors (repeated for Scala 2.11, 2.12, 2.13):

ModifySeqAtOrElseTest.scala:10:18: value atOrElse is not a member of List[A3]
    modify(l1)(_.atOrElse(2, A3(A4(A5("d4")))).a4.a5.name).using(duplicate) should be(l1at2dup)

ModifySeqAtOrElseTest.scala:16:23: value atOrElse is not a member of List[A5]
    modify(items)(_.atOrElse(5, A5("default")).name).using(duplicate)

There is also a secondary error caused by the first: missing argument list for method duplicate (the type inference chain breaks once atOrElse is unknown).

Root cause: The atOrElse extension on Seq/Array focus paths has not been implemented. The test commit (426a6d3) intentionally adds a failing test — the fix (wiring atOrElse into the quicklens macro/path machinery) still needs to be written.