Hardcode constant detection into the cascading compressor

[connortsui20]

Summary

Closes: #8666

The five per-type constant schemes (bool/int/float/string/binary) were "half builtin": they lived in vortex-compressor's builtins module but still had to be registered. Constant handling was also scattered across three places: the compressor's own all-null short-circuit, the per-type constant schemes, and an inline is_constant check in TemporalScheme.

Constant detection is now built directly into CascadingCompressor. A new crate-private constant module detects constant leaves using the cheapest available evidence per type.

Changes in behavior:

• The big change is that constant detection is unconditional:, as it applies even for a compressor constructed with an empty scheme list, and can no longer be disabled via exclude_schemes. I think this is fine because it is essentially never a good idea to not check for a constant array (except maybe for tiny arrays, but I don't think we need to worry about that), and we have no code that tries to exclude constant array.
• Constant detection now covers ALL leaf types uniformly, adding decimal and extension arrays.
• Constant string/binary arrays no longer pay for dict/FSST sampling before the (previously deferred, last-registered) constant check happens.
• Also a small perf change: The old scheme-based path found the first valid element with a per-index is_valid loop that re-derived validity on every call; the new compress_constant function materializes the validity mask once and uses Mask::first.

[codspeed-hq]

Merging this PR will not alter performance

⚡ 5 improved benchmarks
❌ 1 regressed benchmark
✅ 1601 untouched benchmarks
⏩ 42 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_canonical_into[(1000, 10)]	154.7 µs	191 µs	-18.99%
⚡	Simulation	encode_varbin[(1000, 8)]	161.2 µs	140 µs	+15.13%
⚡	Simulation	encode_varbin[(1000, 4)]	159.4 µs	139.1 µs	+14.62%
⚡	Simulation	encode_varbin[(1000, 32)]	164.9 µs	144.8 µs	+13.91%
⚡	Simulation	encode_varbin[(1000, 2)]	156.3 µs	138.5 µs	+12.87%
⚡	Simulation	chunked_varbinview_opt_canonical_into[(100, 100)]	341 µs	305.8 µs	+11.51%

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 claude/hardcode-schemes-compressor-97kru1 (561de5b) with develop (a6e3d3e)}

Footnotes

1. 42 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. ↩