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(