StigSpriteIconValidator

Build-time round-trip check: every StigSpriteIcon entry must resolve to a <symbol id="..."> in the vendored stig-icons.svg. The vendored sprite is generated from pinned Iconify JSON packages by StigIconSpriteGenerator; if the Kotlin side adds an enum value but no one regenerates the sprite, every diagram using that icon would render an empty box. This validator catches the divergence at task-action time instead.

Mirrors the com.stigway.gradle.support.StigIcon.validateAllIcons pattern but applies to the new iconify-driven sprite system (Material Symbols was its predecessor — see issue 15 follow-up "iconify migration 3b/3c" for the full retirement).