add test fixtures for array.concat

[cs01]

Summary

• array.concat() was the only array method with zero test coverage. Adds fixtures for numeric, string, and empty-array edge cases.
• All tests pass including empty-to-empty concat (0-length memcpy).

Test plan

• array-concat.ts — numeric array concat, verifies values and immutability
• array-concat-strings.ts — string array concat
• array-concat-empty.ts — empty array edge cases (concat with empty, empty with non-empty, empty with empty)
• npm run verify:quick passes