Add errdefer allocator.free() for array items and map entries slices in decodeAt so partial allocations are cleaned up on error. Add 3 tests using std.testing.checkAllAllocationFailures to exhaustively verify that every allocation failure in decode (flat map, nested record, array) is handled without leaking — tests use ArenaAllocator over the failing allocator to match the intended usage pattern. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>