refactor(dict): clarify compute invariants

[connortsui20]

Summary

Cleans up dictionary scalar-function pushdown and code selection without changing behavior.

Changes

Clarifies the pushdown guards and unsafe invariants, and documents why take and filter reset all_values_referenced.

[codspeed-hq]

Merging this PR will improve performance by 23.86%

⚠️ Unknown Walltime execution environment detected

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⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
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⚡ 4 improved benchmarks
✅ 2108 untouched benchmarks
⏩ 106 skipped benchmarks¹
🗄️ 4 archived benchmarks run²

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	arrow_checked_add_u32_neon[16384]	20.4 µs	12.4 µs	+65.06%
⚡	WallTime	words_gather_scalar_avx2[65536]	9.5 µs	8.3 µs	+14.6%
⚡	Simulation	compact[(2048, 90)]	1.5 µs	1.4 µs	+11.89%
⚡	Simulation	compress_fsst[(500, 64, 4)]	488.1 µs	439 µs	+11.19%

Tip

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_{Comparing ct/dict-compute-cleanup (0bd99e6) with develop (ee1ac25)}

Footnotes

1. 106 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. 4 benchmarks were run, but are now archived. If they were deleted in another branch, consider rebasing to remove them from the report. Instead if they were added back, click here to restore them. ↩