feat: add async database fiber support

[DaniPopes]

Adds a fiber-backed async database adapter that lets synchronous EVM execution yield while host database futures are pending. The asyncdb feature now wires the fiber stack through the EVM container and exposes async transaction and system-call extension traits while preserving the existing blocking wrapper constructor shape.

[codspeed-hq]

Merging this PR will not alter performance

✅ 176 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing DaniPopes:dani/async-fiber-host-io (b1e7ccc) with main (69b54cc)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩