deb: add -j/--jobs to control parallel build jobs

By default the number of parallel jobs is detected with nproc inside
the build context. Add a -j/--jobs option so an explicit count can be
honored instead, threading it through build_binary_package and
local::build into DEB_BUILD_OPTIONS=parallel=N.

src/deb/local.rs

@@ -37,6 +37,7 @@ pub async fn build(
     inject_packages: Option<&[&str]>,
     ctx: Arc<Context>,
     ui: Option<Arc<DebUi>>,
+    jobs: Option<usize>,
 ) -> Result<(), Box<dyn Error>> {
     let sink: Option<Arc<dyn LineSink>> = ui.as_ref().map(|u| u.sink());
 
@@ -45,21 +46,25 @@ pub async fn build(
     env.insert("LANG".to_string(), "C".to_string());
     env.insert("DEBIAN_FRONTEND".to_string(), "noninteractive".to_string());
 
-    // Parallel building: find local number of cores, and use that
-    let num_cores = ctx
-        .command("nproc")
-        .output()
-        .map(|output| {
-            if output.status.success() {
-                String::from_utf8_lossy(&output.stdout)
-                    .trim()
-                    .parse::<usize>()
-                    .unwrap_or(1)
-            } else {
-                1 // Default to 1 if nproc fails
-            }
-        })
-        .unwrap_or(1); // Default to 1 if we can't execute the command
+    // Parallel building: honor an explicit -j/--jobs count, otherwise detect
+    // the number of cores available inside the build context (nproc).
+    let num_cores = match jobs {
+        Some(j) => j,
+        None => ctx
+            .command("nproc")
+            .output()
+            .map(|output| {
+                if output.status.success() {
+                    String::from_utf8_lossy(&output.stdout)
+                        .trim()
+                        .parse::<usize>()
+                        .unwrap_or(1)
+                } else {
+                    1 // Default to 1 if nproc fails
+                }
+            })
+            .unwrap_or(1), // Default to 1 if we can't execute the command
+    };
 
     // Build options: parallel, disable tests by default
     env.insert(

src/deb/mod.rs

@@ -33,6 +33,7 @@ pub async fn build_binary_package(
     inject_packages: Option<&[&str]>,
     ctx: Option<Arc<Context>>,
     ui: Option<Arc<DebUi>>,
+    jobs: Option<usize>,
 ) -> Result<Vec<PathBuf>, Box<dyn Error>> {
     let result = build_binary_package_impl(
         arch,
@@ -45,6 +46,7 @@ pub async fn build_binary_package(
         inject_packages,
         ctx,
         &ui,
+        jobs,
     )
     .await;
 
@@ -68,6 +70,7 @@ async fn build_binary_package_impl(
     inject_packages: Option<&[&str]>,
     ctx: Option<Arc<Context>>,
     ui: &Option<Arc<DebUi>>,
+    jobs: Option<usize>,
 ) -> Result<Vec<PathBuf>, Box<dyn Error>> {
     let cwd = cwd.unwrap_or_else(|| Path::new("."));
 
@@ -160,6 +163,7 @@ async fn build_binary_package_impl(
                     inject_packages,
                     build_ctx.clone(),
                     ui.clone(),
+                    jobs,
                 )
                 .await?
             }
@@ -414,6 +418,7 @@ mod tests {
             None,
             Some(ctx),
             None,
+            None,
         )
         .await
         .expect("Cannot build binary package (deb)");