Move array serde from VTable to standalone ArrayPlugin impls

[mhk197]

Summary

• Remove the blanket impl<V: VTable> ArrayPlugin for V.
• Remove serialize / deserialize from the VTable trait. The vtable now only covers in-memory behaviour.
• Every serializable encoding implements ArrayPlugin directly on its vtable type (impl ArrayPlugin for Primitive, impl ArrayPlugin for FoR, ...). There are no new plugin structs, and session.arrays().register(X) is unchanged.
• Each impl lives in a plugin.rs child module of the file that holds impl VTable for X, e.g. primitive/vtable/plugin.rs or alp/array/plugin.rs. Because it is a child module, it can use the parent's private metadata types without visibility changes.
• The old serde bodies are ported verbatim, so the wire format is unchanged.

Encodings that were never serializable lose the two methods and get no plugin: Filter, Slice, Shared, Interleave, ScalarFn, ForeignArray, TransposedBool, PiecewiseSequence and the test-harness vtables.

• The standalone DecimalBytePartsPlugin struct is removed. Its v1/v2 serde now lives in impl ArrayPlugin for DecimalByteParts, and initialize registers DecimalByteParts. The bare_vtable_refuses_serde test is deleted because its premise no longer holds.
• ALPPatchedPlugin and BitPackedPatchedPlugin are unchanged apart from delegating to ArrayPlugin::deserialize(&ALP/&BitPacked, ...).

Behaviour changes

• PiecewiseSequence is no longer registered in ArraySession::default(), and TransposedBool is no longer registered by vortex_fastlanes::initialize. Neither could serialize or deserialize before. Serializing them now fails with "not registered for serialization" instead of "does not support serialization", and serialization_is_not_supported is updated to match.
• PythonVTable gets a minimal ArrayPlugin whose serialize returns Ok(None) and whose deserialize bails, the same as before. This keeps vortex._lib.registry.register working.

[codspeed]

Merging this PR will degrade performance by 13.28%

⚠️ 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.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 4 regressed benchmarks
✅ 2057 untouched benchmarks
⏩ 503 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	density_sweep_single_slice[0.9]	32.1 µs	48.3 µs	-33.57%
❌	Simulation	sum_v2_i64	193.9 µs	224.7 µs	-13.7%
❌	Simulation	sum_i64	194 µs	224.4 µs	-13.56%
❌	WallTime	dict_canonicalize_gt_u8_neon[1000000]	488.7 µs	547.2 µs	-10.68%
⚡	WallTime	compare_u64_avx2	4.1 µs	3.7 µs	+10.8%

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 mk/standalone-array-plugins (e50242a) with develop (292cf6f)}

Footnotes

1. 503 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. ↩