Add tests for delimiter and header features

[vmvarela]

Description

The --delimiter / --tsv and --header / -H features (#9, #10) were shipped without dedicated test coverage. Add integration tests that verify these features work correctly and catch regressions.

Acceptance Criteria

• Test --delimiter "|" produces pipe-separated output
• Test --delimiter "\t" and --tsv produce tab-separated output
• Test --header includes column names as the first output row
• Test --header combined with --delimiter / --tsv
• Test default behavior (no flags) still produces CSV without header
• All tests run as part of zig build test

Notes

• Existing integration tests in build.zig (PR ci: add Zig cache and build.zig integration tests #16) can serve as a pattern
• This was identified as tech debt in the Sprint 2 Retrospective