refactor(variable-pool)!: make bootstrap construction explicit

[laipz8200]

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Related Issue

Refs #66

Summary

• move legacy/bootstrap variable-pool assembly into explicit classmethod constructors
• keep VariablePool() focused on normalized state and reject legacy bootstrap kwargs with a clear migration path
• update focused helpers and runtime tests to use the new construction entrypoint

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation