refactor: box large Error variants (Command, Pkgar) to fix result_large_err

Boxing the Command(Command, ExitStatus) variant (204 bytes) and
Pkgar(pkgar::Error) variant (272 bytes) reduces the Error enum from
272+ bytes to ~80 bytes, eliminating all 70 result_large_err warnings.

Changes:
- Error::Command(Command, ExitStatus) → Command(Box<(Command, ExitStatus)>)
- Error::Pkgar(pkgar::Error) → Pkgar(Box<pkgar::Error>)
- Updated 2 construction sites in fs.rs (Command)
- Updated 1 construction site in package.rs (Pkgar)
- Updated 1 From conversion in lib.rs (Pkgar)
- Updated Display match in lib.rs (Command destructuring)

This is a heap allocation on error paths only — zero cost on the happy path.
Box<T> auto-implements Display/Debug when inner type does, so formatting
is unchanged.

Verification: cargo check ✅, cargo test --lib ✅ 38/38.
Clippy: 74 → 4 warnings (70 eliminated).

src/cook/fs.rs

@@ -201,7 +201,7 @@ pub fn run_command(mut command: process::Command, stdout_pipe: &PtyOut) -> Resul
         .map_err(wrap_io_err!("waiting to exit"))?;
 
     if !status.success() {
-        return Err(Error::Command(command, status));
+        return Err(Error::Command(Box::new((command, status))));
     }
 
     Ok(())
@@ -289,7 +289,7 @@ pub fn run_command_stdin(
     let status = child.wait().map_err(wrap_io_err!("Spawning"))?;
 
     if !status.success() {
-        return Err(Error::Command(command, status));
+        return Err(Error::Command(Box::new((command, status))));
     }
 
     Ok(())

src/cook/package.rs

@@ -158,7 +158,7 @@ pub fn package_toml(
             pkgar_core::Packaging::LZMA2 => {
                 let mut size = header
                     .total_size()
-                    .map_err(|e| Error::Pkgar(pkgar::Error::Core(e)))?
+                    .map_err(|e| Error::Pkgar(Box::new(pkgar::Error::Core(e))))?
                     as u64;
                 let entries = package
                     .read_entries()

src/lib.rs

@@ -43,9 +43,9 @@ pub enum Error {
         dst: PathBuf,
         context: &'static str,
     },
-    Command(Command, ExitStatus),
+    Command(Box<(Command, ExitStatus)>),
     Package(pkg::PackageError),
-    Pkgar(pkgar::Error),
+    Pkgar(Box<pkgar::Error>),
     Other(String),
 }
 
@@ -86,7 +86,8 @@ impl Display for Error {
                     source
                 )
             }
-            Error::Command(command, exit_status) => {
+            Error::Command(boxed) => {
+                let (command, exit_status) = &**boxed;
                 write!(
                     f,
                     "Failed to run [{:?}]: exited with status {}",
@@ -174,7 +175,7 @@ impl From<pkgar::Error> for Error {
                 path,
                 context,
             },
-            _ => Error::Pkgar(value),
+            _ => Error::Pkgar(Box::new(value)),
         }
     }
 }