Add OSS-Fuzz compatible libFuzzer targets for high-level API

[s2ongmo]

Add OSS-Fuzz compatible fuzz targets for high-level API

Summary

This PR modernizes the existing fuzzing infrastructure and adds three libFuzzer-compatible fuzz targets covering the core decode, buffer load/iterate, and streaming/seek APIs. The goal is to integrate minimp3 into Google's OSS-Fuzz for continuous fuzzing.

Background

The existing fuzzing/fuzz.c is an AFL-only harness (stdin + __AFL_LOOP). The LIBFUZZER path in minimp3_test.c does not compile (calls 4-parameter decode_file with 6 arguments). This PR adds standalone libFuzzer targets that work with OSS-Fuzz's build system while keeping the existing AFL harness intact.

What's Added

fuzz/fuzz_decode_frame.c — Core frame decoder

• Exercises mp3dec_decode_frame() in a loop over the input buffer
• Covers minimp3.h (Layer 1/2/3 decode, sync, ID3 skip)

fuzz/fuzz_decode_buf.c — Buffer decode + iteration

• Exercises mp3dec_load_buf() and mp3dec_iterate_buf()
• Covers minimp3_ex.h high-level full-buffer APIs

fuzz/fuzz_decode_ex.c — Streaming API with seek

• Exercises mp3dec_ex_open_buf(), mp3dec_ex_read(), mp3dec_ex_seek()
• Covers the streaming/seeking code paths in minimp3_ex.h

fuzz/mp3.dict — MP3 format dictionary (frame sync words, ID3/APE/Xing tokens)

Build

# Example: ASAN + libFuzzer
clang -g -O1 -fsanitize=address -I. -c fuzz/fuzz_decode_frame.c -o fuzz_decode_frame.o
clang++ -g -O1 -fsanitize=address,fuzzer fuzz_decode_frame.o -o fuzz_decode_frame -lm

All three targets build cleanly with ASAN, UBSAN, and MSAN.

Notes

• The existing fuzzing/ directory and minimp3_test.c are untouched.
• Targets use Apache 2.0 headers per OSS-Fuzz convention.
• minimp3 is CC0/public domain, so there is no license conflict.
• I've also identified a memory safety issue in the streaming seek path — will file separately.

Related: #136

[s2ongmo]

Thanks for merging #139 — the fix looks solid. These fuzz targets (fuzz_decode_frame, fuzz_decode_buf, fuzz_decode_ex) were what originally caught that seek truncation bug. Would you be open to merging this PR so the fuzz directory can be used for continuous fuzzing via OSS-Fuzz?