test: increase coverage from 62% to 93%

[vigo]

Summary

• Add comprehensive tests for all major untested code paths across 6 test files (+627 lines)
• Coverage: 62.4% → 92.9% (app package), 61.9% → 92.1% (total)
• No source code changes — all new code is in _test.go files

Test groups added:

• Constructor & flags: NewCLIApplication, parseFlags (version, defaults, custom, invalid limit, chunks bounds)
• Run integration: successful download, version flag, empty URL, 404 resource, multiple files, failed download
• Progress lifecycle: start() → ticker → render() → finish(), multiple entries
• Download error paths: chunked OpenFile error, 500 response, resume with range ignored, cancelled context, chunked+single fallback fail
• Small gaps: rate limiter large buffer cap, large N wait, finalizePart nonexistent path, parsePipe ErrReader, fetchToFile non-206, root URL basename fallback, checkDiskSpace invalid path

Test plan

• go test -race ./... passes
• golangci-lint run ./... — 0 issues
• Coverage verified: 92.9% (target was 80%+)

🤖 Generated with Claude Code

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