Next.js: dedupe dev once-warnings by stable issue key
onceErrorSet stored full Issue objects by object identity. Fresh issue
objects arrive on every subscription emission, so the set never matched
across recompiles: identical once-warnings re-printed forever, and every
Issue object (title, description, filePath) was retained for the process
lifetime. Key the set by the stable getIssueKey content string instead,
which both bounds retention and restores cross-recompile deduplication.
