fix: pad delta remainder with last value

[rapour]

Summary

Fixes #9556

Changes

Delta encoding remainder is padded with the last value instead of zeros.

[rapour]

Updated the following to make the CI green:

• Linting issues addressed
• The remainder is padded with the last value, so those positions hold real (if synthetic) data. The validity bitmap is transposed separately, and the bit transpose is not the same permutation as the integer transpose — so zero-filling the pad bits marks physical slots that hold real residuals as null. Bitpacking then skips patches for those values and decompression yields wrong results. Padding the mask with true keeps every real slot valid; the pad bits are sliced off by logical_len. Will be addressed fully in a separate PR.
• Delta is now picked in scenarios where it would previously skipped so the snapshots are updated. There is only one case where compression is not improved which is compared to pco. This is due to the fact Delta scheme estimation does not include bases in the cost. Will be addressed in a follow-up PR.

[codspeed-hq]

Merging this PR will degrade performance by 10.85%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 1 regressed benchmark
✅ 1980 untouched benchmarks
⏩ 54 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	compress_fsst[(500, 64, 8)]	530.5 µs	595 µs	-10.85%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing rapour:fix/delta-span (0533a7a) with develop (e4b3421)}

Footnotes

1. 54 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩