Use cursor copies for PiecewiseSequence slices

[danking]

Summary

• add a shared checked cursor-copy helper for PiecewiseSequence slice gathering
• replace extend_from_slice in the primitive, decimal, VarBinView, and VarBin PiecewiseSequence consumers
• keep the safety checks explicit: each copy verifies the destination cursor range against the declared output length and capacity, and callers verify the final cursor before set_len

Validation

• cargo +nightly fmt --all
• cargo clippy -p vortex-array --all-targets --all-features -- -D warnings
• cargo bench -p vortex-array --bench listview_rebuild
• cargo bench -p vortex-array --bench cast_decimal
• cargo test -p vortex-array piecewise_sequence

[codspeed-hq]

Merging this PR will regress 1 benchmark

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 1795 untouched benchmarks
⏩ 52 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	take_fsl_u32_random[16, 1000]	126.9 µs	141.8 µs	-10.51%
⚡	Simulation	copy_nullable[65536]	1.4 ms	1 ms	+32.03%
⚡	Simulation	copy_non_nullable[65536]	1,089.5 µs	908.9 µs	+19.87%

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 codex/piecewise-sequence-cursor-copy (534b70f) with codex/piecewise-sequence-varbin-take (5924fe3)²}

Footnotes

1. 52 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. ↩

2. No successful run was found on codex/piecewise-sequence-varbin-take (79d9e10) during the generation of this report, so 24733c2 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[robert3005]

I would benchmark having *mut T that you keep advancing for every value written so you always hold the next empty slot in that pointer. That's what we do for buffer extend