Complete StructuredData parity and parser hardening (#37)

fuzz/corpus/protobuf_osd/README.md

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+# Protobuf OSD malformed corpus
+
+Each named seed is hex-encoded. The normal Rust hardening suite checks every
+seed and deterministic mutations of a valid all-variant wire document without
+requiring an external fuzzing runtime.

fuzz/corpus/protobuf_osd/malformed.hex

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+empty:
+truncated_varint:80
+zero_field:00
+wrong_type_wire:0d00000000
+oversized_length:2affffffff0f
+truncated_fixed64:210000
+unknown_group:0b
+bad_uuid:0805320100