mux: allow build-time version label override (FREEMKV_BUILD_LABEL)

build.rs emits FREEMKV_VERSION = FREEMKV_BUILD_LABEL when that env is set
(non-empty), else the Cargo package version. The muxing/writing-application
field and the FVI generator tag use it, so a pre-release/test build can be
stamped without bumping Cargo.toml and disturbing the tag-pinned [patch]
version matching. The git short hash is still appended either way.

build.rs

@@ -57,6 +57,18 @@ fn main() {
 /// leaving just the package version. Always emitted so `env!("GIT_SUFFIX")`
 /// resolves on every target.
 fn emit_git_suffix() {
+    // Version label for the muxing-app / FVI generator tag. `FREEMKV_BUILD_LABEL`
+    // overrides the Cargo package version when set (non-empty) — used to stamp a
+    // pre-release/test build without bumping Cargo.toml and disturbing the
+    // tag-pinned [patch] version matching. Unset → the package version.
+    let version = std::env::var("FREEMKV_BUILD_LABEL")
+        .ok()
+        .filter(|s| !s.trim().is_empty())
+        .or_else(|| std::env::var("CARGO_PKG_VERSION").ok())
+        .unwrap_or_default();
+    println!("cargo:rustc-env=FREEMKV_VERSION={version}");
+    println!("cargo:rerun-if-env-changed=FREEMKV_BUILD_LABEL");
+
     let suffix = git_short_hash()
         .map(|h| format!(" (g{h})"))
         .unwrap_or_default();

src/mux/mkv.rs

@@ -774,7 +774,7 @@ 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!("CARGO_PKG_VERSION"), env!("GIT_SUFFIX"));
+            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)?;
         if let Some(t) = title {

src/mux/videomap.rs

@@ -37,7 +37,7 @@ pub const FVI_VERSION: u32 = 1;
 
 /// Producing tool tag for the header `"generator"` member
 /// (`docs/FVI_FORMAT.md` §6).
-pub const FVI_GENERATOR: &str = concat!("freemkv/", env!("CARGO_PKG_VERSION"), env!("GIT_SUFFIX"));
+pub const FVI_GENERATOR: &str = concat!("freemkv/", env!("FREEMKV_VERSION"), env!("GIT_SUFFIX"));
 
 /// Header `"timescale"` for all `pts`/`dts` ticks (`docs/FVI_FORMAT.md` §10).
 /// The highway carries presentation timestamps in nanoseconds, so the timescale