layout reader ctx

[onursatici]

Summary

Introduce LayoutReaderContext, a typed-data registry threaded through reader construction so an ancestor layout can publish values that descendant layouts look up by type at construction time.

Breaking: Layout::new_reader and VTable::new_reader gain a &LayoutReaderContext parameter. Out-of-tree impls will get a compile error and must add it

[codspeed-hq]

Merging this PR will degrade performance by 16.59%

⚠️ 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.

❌ 1 regressed benchmark
✅ 1236 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_opt_canonical_into[(1000, 10)]	187.7 µs	225.1 µs	-16.59%

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 os/layout-overrides (e8f8e0d) with develop (008c1d9)}