fuzzing: add cjson_mutate_fuzzer for Duplicate/Compare/Replace/Insert APIs

[XananasX7]

Summary

The existing cjson_read_fuzzer covers cJSON_ParseWithOpts, cJSON_Print, cJSON_PrintBuffered, and cJSON_Minify. The following APIs have no OSS-Fuzz coverage:

API	Description
cJSON_Duplicate(item, recurse)	Deep/shallow recursive copy of a JSON tree
cJSON_Compare(a, b, case_sensitive)	Deep equality comparison
cJSON_ParseWithLengthOpts	Length-bounded parse (no NUL needed)
cJSON_InsertItemInArray	Mid-array insertion with item shift
cJSON_DetachItemFromArray	Detach item and re-insert
cJSON_ReplaceItemViaPointer	In-place item replacement

New harness: fuzzing/cjson_mutate_fuzzer.c

Uses a single fuzz-byte to select which operation sequence to execute:

• op 0 – Duplicate(recurse=1) → Compare for equality
• op 1 – Build array, InsertItemInArray at position 0, DetachItemFromArray
• op 2 – ReplaceItemViaPointer on root's first child
• op 3 – ParseWithLengthOpts with a deliberately short length bound
• op 4 – Duplicate(recurse=0) (shallow) → PrintUnformatted
• op 5 – Compare root against itself (both case modes)

Compiles cleanly with clang -fsyntax-only.