Complete StructuredData parity and parser hardening (#37)

fuzz/corpus/notation_llsd/README.md

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+# Notation LLSD malformed corpus
+
+Each line in `malformed.txt` has a seed name followed by the raw notation input.
+The normal Rust hardening suite executes every seed and deterministic mutations
+of a valid all-variant document under no-panic and resource-limit checks.

fuzz/corpus/notation_llsd/malformed.txt

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+empty:
+unknown:?
+unterminated_string:s(10)"x
+missing_array_end:[i1
+missing_map_value:{'key':
+bad_base64:b64"***"
+bad_date:d"not-a-date"