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/main.rs

@@ -78,6 +78,7 @@ fn main() {
                     .long_help("Cross-compile for target architecture (instead of using qemu-binfmt)\nNote that most packages cannot be cross-compiled").required(false))
                 .arg(arg!(--mode <mode> "Change build mode [local]").required(false)
                     .long_help("Change build mode [local]\nDefault will chose depending on other parameters, don't provide if unsure"))
+                .arg(arg!(-j --jobs <jobs> "Number of parallel build jobs (default: number of CPUs available in the build context)").required(false))
                 .arg(arg!(--verbose "Show raw tool output instead of the live build view").required(false)
                     .long_help("Show raw tool output instead of the live build view.\nAlso implied by RUST_LOG=debug for pkh's own logs.")),
         )
@@ -313,6 +314,14 @@ fn main() {
                 .copied()
                 .unwrap_or(false);
 
+            let jobs = sub_matches.get_one::<String>("jobs").map(|s| s.as_str());
+            let jobs = jobs.map(|j| {
+                j.parse::<usize>().unwrap_or_else(|_| {
+                    error!("Invalid --jobs value '{}': expected a positive integer", j);
+                    std::process::exit(1);
+                })
+            });
+
             // Live build view: disabled by --verbose or when stdout is not a
             // terminal (DebUi handles the non-TTY case itself)
             let ui = if verbose {
@@ -333,6 +342,7 @@ fn main() {
                     inject_packages,
                     None,
                     ui.clone(),
+                    jobs,
                 )
                 .await
             });