schemagen: support objects with more than 32 properties

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

contrib/schemagen/README.md

@@ -73,3 +73,18 @@ union `type` lists other than `["T", "null"]`, non-string enums, and remote
   cannot hold partial digits.
 - `test_gen.cpp` generates from `example.schema.json` and exercises the parser
   byte-by-byte (covering chunked strings/numbers), plus the rejection cases.
+
+## Testing
+
+After building weaseljson (e.g. `cmake -S . -B build && make -C build`), run:
+
+```sh
+cd contrib/schemagen
+./run_tests.sh
+```
+
+This regenerates the example parser (`gen.h`) and a regression parser with 40
+required properties (`big.h`), compiles `test_gen.cpp` and `test_big.cpp`, and
+runs both. `test_big.cpp` specifically covers issue #3: it checks that a
+40-property object accepts all fields, rejects a missing field at index 32, and
+rejects duplicate keys around the 32-bit boundary.