Harden indexed lane sources

[connortsui20]

Rationale for this change

Lets indexed lane sources return borrowed or otherwise non-Copy items. RowFn needs this for typed row inputs without adding a second kernel abstraction.

• Progress towards: Tracking Issue: Define the RowFn API #9129

What changes are included in this PR?

Removes the Copy bound from IndexedSource::Item. It also makes LaneZip fields private, validates lengths once in LaneZip::new, and removes the repeated assertion from its inline len method.

This is a prerequisite only. The first production RowFn adopter is in #9345, so there is no meaningful RowFn performance comparison at this layer.

What APIs are changed? Are there any user-facing changes?

LaneZip must now be constructed with LaneZip::new instead of tuple syntax. The workspace has no direct field construction outside this module.

[connortsui20]

Another fun thing I found in my rustc investigation: when removing the Copy bound, sometimes the compiler can generate faster code because instead of a copy (load and store), it performs a move, which sometimes can allow a loop to vectorize better? Why? I have no idea! And when I tried to minimize for a repro I couldn't find a good one because things kept changing between rust versions.

Regardless, it seems like we should just take of the Copy constraint because it's not needed.

[codspeed-hq]

Merging this PR will degrade performance by 1.6%

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 1960 untouched benchmarks
⏩ 89 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cold_misaligned[(64, 256)]	4.4 ms	5 ms	-12.5%
⚡	Simulation	chunked_varbin_opt_to_varbinview_builder[(10, 100)]	630 µs	569.3 µs	+10.66%

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 ct/lane-kernel-sources (ea0f552) with develop (402a5ac)}

Footnotes

1. 89 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. ↩