mux: single source of truth for the version label (VERSION_LABEL/MUX_APP)

The CLI --version, the MKV muxing/writing-application field, and the FVI generator
all derive from one libfreemkv const, so a binary reports the exact same label it
stamps into the files it produces — no split-brain where an MKV claims one version
and the binary another.

src/lib.rs

@@ -85,6 +85,20 @@
 //! | E8xxx | Keydb errors (fetch, parse, load) |
 //! | E9xxx | Stream / mux errors (URL, PES, pipeline) |
 
+/// Single source of truth for every freemkv version surface.
+///
+/// `FREEMKV_VERSION` is the package version, overridable at build time via the
+/// `FREEMKV_BUILD_LABEL` env (see `build.rs`); `GIT_SUFFIX` is the git short
+/// hash. The CLI's `--version`, the MKV muxing/writing-application field, and
+/// the FVI generator tag all derive from these two consts, so a binary reports
+/// the exact same label it stamps into the files it produces — no split-brain
+/// where an MKV claims one version and the binary another.
+pub const VERSION_LABEL: &str = concat!(env!("FREEMKV_VERSION"), env!("GIT_SUFFIX"));
+
+/// The muxing/writing-application string written into MKV output
+/// (`"freemkv <version> (g<hash>)"`).
+pub const MUX_APP: &str = concat!("freemkv ", env!("FREEMKV_VERSION"), env!("GIT_SUFFIX"));
+
 pub mod aacs;
 pub(crate) mod clpi;
 pub mod consts;

src/mux/mkv.rs

@@ -773,10 +773,8 @@ impl<W: Write + Seek> MkvMuxer<W> {
         }
         // Stamp the freemkv version so any muxed file is traceable to the build
         // that produced it (MediaInfo "Writing application"/"library").
-        const FREEMKV_MUX_APP: &str =
-            concat!("freemkv ", env!("FREEMKV_VERSION"), env!("GIT_SUFFIX"));
-        ebml::write_string(&mut writer, ebml::MUXING_APP, FREEMKV_MUX_APP)?;
-        ebml::write_string(&mut writer, ebml::WRITING_APP, FREEMKV_MUX_APP)?;
+        ebml::write_string(&mut writer, ebml::MUXING_APP, crate::MUX_APP)?;
+        ebml::write_string(&mut writer, ebml::WRITING_APP, crate::MUX_APP)?;
         if let Some(t) = title {
             ebml::write_string(&mut writer, ebml::TITLE, t)?;
         }