Replace the 32-bit `uint32_t seen` bitmask with a `std::vector<uint64_t>`
bitset sized to the actual field count. `requiredMask` is now a vector of
the same word count, and required-field checks use per-word masking so that
optional fields do not cause false rejections.
Adds big.schema.json + test_big.cpp regression tests covering the issue
reproducer (40 required properties, missing/duplicate at index 32), plus
run_tests.sh to exercise both test_gen.cpp and test_big.cpp.
Fixes#3
