Summary

• Replace per-element XDR library vtable dispatch (xdrmem_create/xdr_int/xdr_double/xdr_destroy) with an inline R_swap32 byte-swap for bulk integer, real, and complex vector serialization and deserialization, enabling compiler autovectorization of the inner loops
• For deserialization (InIntegerVec, InRealVec, InComplexVec), read directly into the target SEXP buffer and byte-swap in place, eliminating the static intermediate buf
• Buffer character output in OutString to reduce per-character OutBytes calls
• Remove redundant sscanf into a second buffer in InInteger and InReal — InWord already provides the parsed token
• Increase reference hash table size from 1099 to 6151 to reduce collisions
• Cache strlen(PRIMNAME(s)) to avoid computing it twice in WriteItem

Test plan

• Verify make check passes (serialization round-trips are exercised extensively by the test suite)
• Benchmark serialize()/unserialize() on large integer, double, and complex vectors to confirm performance improvement
• Test on both little-endian (x86_64, aarch64) and big-endian targets to verify WORDS_BIGENDIAN paths