fix[bitpacked]: slice patches in execute method (not reduce).

[joseph-isaacs]

Problem: BitPacked's SliceReduce implementation called patches.slice(), which reads buffers (search_index does binary search on buffer data, execute_scalar executes on chunk offsets). This violates the SliceReduce contract that requires metadata-only operations.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ 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.

✅ 1208 untouched benchmarks

---

_{Comparing ji/fix-up-slice-bp-reduce (5dbe242) with develop (fb2feb2)}