[Neo-matrix-coder]

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:

This is a pure C-to-Rust port preserving existing behavior (no bug fix sample required). You can verify the port and run all unit tests via:

cargo test --manifest-path src/rust/lib_ccxr/Cargo.toml
cargo test --manifest-path src/rust/Cargo.toml

---

Summary of Changes

This PR ports get_file_extension, get_basename, and create_outfilename from src/lib_ccx/utility.c to idiomatic Rust in lib_ccxr and exposes C-FFI bindings in ccx_rust, following the 5-step workflow in docs/Rust_migration_guide.md:

1. Idiomatic Rust (src/rust/lib_ccxr):
   • Added #[repr(u32)] and FromRepr to OutputFormat in src/rust/lib_ccxr/src/common/constants.rs and implemented OutputFormat::extension(&self) -> Option<&'static str>.
   • Implemented get_basename (preserving directory prefixes while stripping the final extension after the last path separator) and create_outfilename in src/rust/lib_ccxr/src/util/mod.rs.
   • Updated the C-to-Rust conversion tables in module documentation.
2. C-FFI Layer (src/rust/src/libccxr_exports/util.rs):
   • Implemented ccxr_get_file_extension returning pointers from static null-terminated C-string literals (c"...").
   • Implemented ccxr_get_basename and ccxr_create_outfilename with # Safety documentation, null-pointer handling, and valid heap allocation for empty results.
3. C Integration (src/lib_ccx/utility.c):
   • Connected get_file_extension, get_basename, and create_outfilename to their ccxr_* equivalents inside #ifndef DISABLE_RUST blocks (prototypes declared alongside each wrapper), preserving the original C fallback under #else.
4. Testing:
   • Added unit tests in lib_ccxr and libccxr_exports covering all 17 OutputFormat variants, invalid discriminants, relative/absolute/Windows paths, compound extensions, hidden files, empty strings, and null pointers.

[Neo-matrix-coder]

CI note: the 7 Linux jobs fail at Install dependencies (libgpac-dev has no installation candidate on the runner), before this PR's code is checked out. The Format sourcecode / format job reports clang-format diffs only in ccx_decoders_isdb.c and mp4.c, which this PR doesn't modify. Rust fmt/clippy, test_rust, and the macOS, Docker and Windows builds pass. The same failures show up on other open PRs (e.g. #2436, #2435).