Tighten chart enrichment and type inference boundaries

Problem

Chart enrichment and type inference still cross module boundaries in ways that blur responsibilities between render-time orchestration and spec generation. Decisions should clearly fill gaps rather than behave like an implicit second rendering layer, and shared type inference should not live inside the main Vega-Lite module. Refine the enrichment gate, move type inference into a neutral helper module, and add tests that lock in the intended fill-only behavior.

Context

Possible Solutions

Plan

Implementation Progress

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