V8 external-memory retainer analyzer

Chose a streaming C++ parser with fixed-size node records, a string-offset index into the unchanged snapshot, and externally sorted reverse edges over wholesale JavaScript/Python parsing because an 800 MB V8 JSON snapshot expands far beyond its file size and can endanger OpenCode. Use bounded sort runs, count weak edges in aggregates, but exclude weak edges from GC-root retaining paths because they do not keep targets alive.