unify-warning-prefix-casing.md

@arkenv/build	patch
@arkenv/nextjs	patch
@arkenv/nuxt	patch

Standardize warning and error log prefix formatting

Introduce a shared log.ts utility module in @arkenv/build with unified prefix constants and helper functions (logBuildWarning, logBuildError, formatBuildError, logWatcherError). Update @arkenv/nextjs and @arkenv/nuxt to use these helpers instead of manually-prefixed string literals, eliminating casing inconsistencies and code duplication.