Tags with non-ASCII characters (Hebrew, Arabic, CJK, emoji) can crash ISR
responses with ERR_INVALID_CHAR when written to x-next-cache-tags, and
silently break invalidation when storage form diverges from the form
revalidateTag/revalidatePath produce.
Add encodeCacheTag/encodeCacheTags helper (matches behavior of
vercel/next.js#93601) and apply it at every public boundary: cacheTag,
revalidateTag, revalidatePath, updateTag, unstable_cache options.tags,
fetch next.tags, and the path-derived _N_T_ tag builders.
Closes#1138
James Anderson