Fix dd audit test failures

[kelp]

Summary

• Fix conv=swab to preserve the last byte on odd-length input instead of zeroing it (GNU dd behavior)
• Fix conv=sync to pad with spaces instead of NUL when conv=block or conv=unblock is active
• Replace both ascii_to_ebcdic and ascii_to_ibm tables with correct GNU coreutils values (the entire high half 0x80-0xFF was wrong, causing incorrect EBCDIC conversions for extended characters and wrong mappings for '^' and '~')
• Update existing swab odd-length test to match corrected GNU behavior

Test plan

• All 4 previously failing dd audit tests now pass
• All existing dd tests continue to pass (40/40)
• No new test failures introduced

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