refactor: isolate bitpacked v1 serde in the plugin

[mhk197]

Move serialization and deserialization of fastlanes.bitpacked into BitPackedPlugin, shared with the external-patches adapter. The plugin owns the protobuf metadata and wire validation.

The VTable retains only the required error-returning serde methods and in-memory array validation.

[codspeed]

Merging this PR will regress 1 benchmark

⚠️ 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.

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 2250 untouched benchmarks
⏩ 218 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	mul_i32_nonnull_avx512	7.1 µs	7.9 µs	-10.38%
⚡	WallTime	filtered_sink_i64_avx2[OneNullInEight]	25.8 µs	23.1 µs	+11.35%
⚡	WallTime	mul_u32_nonnull_avx512	6.3 µs	5.6 µs	+11.25%

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/bitpacked-stack-01-wire-boundary (9681199) with develop (c6678f3)}

Footnotes

1. 218 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. ↩