Complete StructuredData parity and parser hardening (#37)

fuzz/corpus/json_osd/README.md

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+# JSON OSD malformed corpus
+
+The normal Rust hardening suite feeds these parser-edge cases and deterministic
+mutations of a valid all-variant JSON document to the bounded JSON adapter.

fuzz/corpus/json_osd/malformed.txt

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+empty:
+trailing:[]x
+unterminated_string:"x
+unterminated_array:[1
+unterminated_object:{"x":1
+duplicate_property:{"x":1,"x":2}
+non_finite_number:1e99999