[cfsmp3]

In raising this pull request, I confirm the following (please check boxes):

Reason for this PR:

• This PR adds new functionality.
• This PR fixes a bug that I have personally experienced or that a real user has reported and for which a sample exists.
• This PR is porting code from C to Rust.

Sanity check:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• If the PR adds new functionality, I've added it to the changelog. If it's just a bug fix, I have NOT added it to the changelog.
• I am NOT adding new C code unless it's to fix an existing, reproducible bug.

Repro instructions:

Any macOS CMake build on the current macos-26-arm64 runner image (e.g. the cmake, cmake_ocr_hardsubx and cmake_ffmpeg_mp4 jobs of "Build CCExtractor on Mac", failing on master since Oct 15) stops at the final link:

ld: multiple errors: tapi error: malformed file
/Library/Developer/CommandLineTools/SDKs/MacOSX27.0.sdk/usr/lib/libiconv.2.tbd:4:20: error: unknown architecture
 in '/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libiconv.tbd'

---

Corrosion hard-codes /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib as an INTERFACE_LINK_DIRECTORIES entry of the Rust static library on macOS (Corrosion.cmake, still present in Corrosion 0.6.1, so upgrading doesn't help). The runner image of 2026-10-13 installed a macOS 27 SDK and repointed that MacOSX.sdk symlink to it, while the selected toolchain is still Xcode 26.6 (SDK 26.5). Compilation uses the Xcode SDK, but the explicit -L makes Xcode 26's ld pick up the 27 SDK's .tbd stubs, which it can't parse.

The libraries rustc asks for (-lSystem -lc -lm) resolve through the compiler's own SDK without that -L — the shell and autoconf builds on the same runner link fine — so this clears the property on Apple after corrosion_import_crate. Linux and Windows are unaffected (the block is if(APPLE)).

Verified on the macOS runners: all 9 jobs of "Build CCExtractor on Mac" pass on this branch, including the three CMake jobs. Diagnosis (runner SDK layout, the CMake link script showing the stray -L) came from a throwaway CI branch.