Add ASan flag propagation for fuzz testing

[alejandroechev]

Summary

When building under cargo-fuzz, propagate AddressSanitizer flags to the CMake C build so ASan can detect memory errors (buffer overflows, use-after-free, etc.) in the native Opus C code when fuzz testing through Rust FFI wrappers.

Changes

Single change in build.rs: detect CARGO_CFG_FUZZING (set automatically by cargo-fuzz) and pass -fsanitize=address, -fno-omit-frame-pointer, and -g to the CMake C compiler.

Impact

• Zero impact on normal builds - flags only activate when CARGO_CFG_FUZZING is set
• Enables ASan in fuzz testing - memory errors in libopus C code are now detectable

Validation

Tested in the media_components nightly pipeline by injecting an intentional heap-buffer-overflow in opus_decode_float and confirming ASan caught it with full source attribution:

==17998==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200000178c
WRITE of size 1 at 0x60200000178c thread T0
    #0 opus_decode_float opus_decoder.c:1000:12

Pipeline run: https://skype.visualstudio.com/SCC/_build/results?buildId=77788743