Add shared anchor transform helper scaffold for placement refactor

[tracygardner]

Summary

• add a new api/anchorTransform.js module to centralize anchor/pivot coordinate math
• expose reusable helper methods:
  • getPivotSettings(mesh)
  • getLocalAnchorOffset(mesh, pivotSettings)
  • resolveWorldAnchor(mesh, pivotSettings)
  • setMeshByWorldAnchor(mesh, target, options)
• wire the module into flock.js so helpers are available on the shared flock object and receive the flock reference

Why

This is the first incremental step of the single coherent refactor to move away from scattered manual offset calculations while preserving current behavior. It provides a canonical helper surface without changing existing transform callsites yet.

Notes

• no behavior migration is included in this commit; this is a scaffold step only
• existing repo lint issues remain outside this change and are unchanged

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Codex Task