docs: show decimal encoding evolution

[connortsui20]

Summary

Addresses the diagram feedback and suggested before/after comparison from #9947. Shows how adding multi-child decimal support preserves the v1 wire contract while adding v2.

Changes

Separates the implementations before and after multi-child support, labels their read/write paths, and removes the unrelated footer details. Updates the example and caption to distinguish the implementation change from serialization.

[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.

⚠️ 3 benchmarks measured no execution time

Nothing ran under measurement, usually because the compiler removed the code under test. These results are not comparable, so they count as unchanged.

Preventing compiler optimizations

⚡ 6 improved benchmarks
❌ 1 regressed benchmark
✅ 2225 untouched benchmarks
⏩ 329 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	take_fsl_random[128, 10]	32.7 µs	58.6 µs	-44.17%
⚡	Simulation	take_fsl_u32_random[128, 100]	137.4 µs	76.9 µs	+78.59%
⚡	Simulation	take_fsl_u64_random[64, 10]	53.7 µs	30.8 µs	+74.42%
⚡	Simulation	take_fsl_f16_random[256, 100]	141.2 µs	87.9 µs	+60.74%
⚡	Simulation	density_sweep_single_slice[0.9]	45.2 µs	28.5 µs	+58.47%
⚡	Simulation	take_fsl_random[64, 100]	143.9 µs	124.9 µs	+15.15%
⚡	WallTime	filtered_owned_i64_avx512[OneNullInEight]	26 µs	23.4 µs	+11.26%
⚠️	Simulation	fixed_16_advancing_ptr_safe[100]	< 1 ns	< 1 ns	N/A
⚠️	Simulation	preverify_advancing_ptr_unchecked[1000]	< 1 ns	< 1 ns	N/A
⚠️	Simulation	preverify_advancing_ptr_unchecked[10000]	< 1 ns	< 1 ns	N/A

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/fix-decimal-diagram (306608c) with develop (2c8c7ee)²}

Footnotes

1. 329 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. No successful run was found on develop (9e8abca) during the generation of this report, so 2c8c7ee was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩