Address review feedback: remove proofs, revert external libs, make checks portable

- Remove .prime-research/ and formal_proofs/ from git repository (per @arnaud-lb).
- Revert ext/date/lib/parse_tz.c (external timelib dependency).
- Revert ext/fileinfo/libmagic/cdf.c (unreachable bounds in libmagic).
- Revert Zend/zend_types.h (nTableSize is already bounded by HT_MAX_SIZE).
- Revert Zend/zend_multiply.h fallback block.
- Replace non-portable __builtin_add_overflow in main/streams/memory.c with portable standard C unsigned boundary checks (fixes Windows MSVC build).

Author: mkrishna793