[professor-moody]

Summary

Add defensive bounds checks and malformed-schema stability validation to BinaryAnnotator in src/binary_annotator.h and src/binary_annotator.cpp, while preserving normal annotation layouts.

• Handle empty region lists before accessing the last region in FixMissingSections() and ContainsSection().
• Keep inline struct traversal state inside BuildStruct() entry and exit. An active-object stack stops cyclic traversal, and a scope guard restores that stack on exit. A per-root stop flag prevents further descent after a cycle, exhausted input, or a non-progressing child. The fixed-array call block and function signatures remain unchanged.
• Validate Vector64 byte spans using remaining bytes and division, and preserve the vector section type when reporting an invalid length.
• Read union type-vector offsets using the field's 32-bit or 64-bit width, validate the resulting data position, and advance the element slot when recording an invalid union-vector element.

Validation

BinaryAnnotatorBoundsTest(), BinaryAnnotatorEmptySectionTest(), and BinaryAnnotatorUnionOffset64Test() in tests/fuzz_test.cpp run through the existing flattests suite. CMake and Bazel test wiring include the annotator implementation.

• Positive controls cover valid Vector64, empty vectors, a verified union vector, nested inline arrays, sibling traversal, and repeated annotation calls on the same instance.
• Existing malformed-schema and buffer regressions cover cyclic structs, empty sections, vector bounds, and union-element bookkeeping. A small truncated nested-array control checks that annotation growth stops when its data ends.
• The focused empty-section test checks that a later string is still annotated after an empty union section, including a repeated annotation call. The 64-bit union test checks the eight-byte type-vector prefix, an out-of-range full-width offset whose low 32 bits are in range, and a truncated eight-byte companion offset.
• Temporary counters confirmed that the empty-region lookup and 64-bit union offset paths execute. All counters were removed before the final builds.
• The patched full suite prints ALL TESTS PASSED and exits 0 under AddressSanitizer and UndefinedBehaviorSanitizer with ASAN_OPTIONS=detect_leaks=0 UBSAN_OPTIONS=halt_on_error=1. Leak detection is disabled for the unchanged cross-namespace fixture documented by the prior independent review; address and undefined behavior checks remain enabled.
• The patched Release suite exits 0. The existing annotation generator exits 0 in both builds and leaves all 24 tracked .afb files unchanged: 23 regenerated outputs and one historical file.
• Changed C++ ranges have zero clang-format replacements and added lines are at most 80 columns. Bazel wiring was inspected but Bazel was not executed.

The changes do not alter the wire format or public function signatures. Inline struct traversal has no new fixed-depth cutoff.