Updated MAC Core helper output contract

Property	Value
description	Froze plain, HTML, and machine-output behavior for the shipped helper surface.
version	1.0.0
type	Update
related	Utils, Format Datetime, Helpers

• Kept plain text as the default output for helpers that support multiple return modes.
• Standardized helper HTML output on fixed mac-core-* classes instead of leaving a supported classless or caller-defined class-map mode in the stable contract.
• Froze mac_core_format_datetime() around preset and view-driven plain, HTML, diff, timezone, and attr output modes.
• Added plain, html, and raw return modes to mac_core_format_price() as part of the stable contract.
• Kept scalar helpers typed and non-HTML, and kept label helpers plain-string only.