Extract shared Vega-Lite spec builder helpers

ID	DFT_CORE-EXTRACT_SHARED_VEGA_LITE_SPEC_BUILDER_HELPERS
Status	completed
Priority	p2
Milestone	m1-ft-analytics-analyst-pilot
Owner	sr-engineer-architect

Problem

Chart-specific Vega-Lite generators still repeat the same spec assembly patterns for title handling, dimensions, tooltip entries, and field normalization across histogram, arc, rect, boxplot, error, and layered charts. This duplication increases maintenance cost and makes config-driven defaults easier to drift. Introduce shared helper builders so chart-type functions become small, mechanical translations from normalized chart definitions to Vega-Lite specs.

Extract shared Vega-Lite spec builder helpers

Problem

Context

Possible Solutions

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