feat: design doc for kona prestate native build

[ajsutton]

Summary

• Design doc for refactoring the kona prestate build to support both native (non-Docker) and reproducible Docker paths from a single definition
• Extracts build logic from Dockerfile stages into just targets in rust/justfile
• Reduces cannon-builder Docker image to apt packages only; tools installed via mise from shared mise.toml
• Follows the proven pattern from op-program/Dockerfile.repro + op-program/repro.justfile

Test plan

• Review design doc for completeness and accuracy
• Validate proposed architecture against current kona prestate build
• Confirm CI impact analysis covers all affected jobs

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