arch: migrate to uucore for true uutils drop-in compatibility

Fixes #8

- Added uucore 0.7 as workspace dependency
- main.rs now uses uucore::bin!(uu_passwd) one-liner (SIGPIPE,
  localization, signal handling all handled by uucore)
- uumain decorated with #[uucore::main], returns UResult<()>
- Created PasswdError enum implementing UError trait with correct
  exit code mapping (1=permission, 2=options, 3=failure, 4=missing,
  5=busy, 6=invalid, 10=PAM)
- Internal helpers return Result<T, PasswdError> instead of raw i32
- Informational messages use uucore::show_error! macro (auto-prefixes
  with utility name)
- Error propagation via ? operator instead of early return with codes

The tool now follows the exact uucore API contract:
  pub fn uumain(args: impl uucore::Args) -> UResult<()>
  pub fn uu_app() -> Command

95 tests passing on Debian/Alpine/Fedora, zero clippy warnings.

Cargo.toml

@@ -31,6 +31,9 @@ shadow-core = { path = "src/shadow-core" }
 # CLI
 clap = { version = "4", features = ["derive", "wrap_help"] }
 
+# uutils integration
+uucore = "0.7"
+
 # Error handling
 thiserror = "2"
 

src/uu/passwd/Cargo.toml

@@ -20,6 +20,7 @@ libc = { workspace = true }
 nix = { workspace = true }
 shadow-core = { workspace = true, features = ["shadow", "login-defs"] }
 thiserror = { workspace = true }
+uucore = { workspace = true }
 
 [features]
 default = []

src/uu/passwd/src/main.rs

@@ -3,7 +3,4 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-fn main() {
-    let code = uu_passwd::uumain(std::env::args_os());
-    std::process::exit(code);
-}
+uucore::bin!(uu_passwd);