feat(core): reject root composer.json that requires its own name

Mirrors Composer\Package\Loader\RootPackageLoader::load(): if the root
package's "name" appears as a key in its own "require" or "require-dev"
map, fail loudly before reaching the resolver. Without this, Mozart
would silently let the request hit Packagist (which has no entry for
the root's vendor/name) and report a misleading "could not be found"
error.

Wired into install::execute (when a lock file is present) and
update::execute (the no-lock fallback path). Carries the same wording
as Composer's RuntimeException so a future EXPECT-OUTPUT comparison
will match.

Also extends the installer test harness: when a fixture sets
EXPECT-EXCEPTION but no EXPECT-EXIT-CODE, assert that Mozart exits
non-zero. Full exception-class matching remains a follow-up (see
.ken/test_design.md §7.2).

Closes the gap exercised by the install-self-from-root installer
fixture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

3 files changed, 37 insertions, 13 deletions

diff --git a/crates/mozart/src/commands/install.rs b/crates/mozart/src/commands/install.rs
index f920f02..b9ff1af 100644
--- a/crates/mozart/src/commands/install.rs
+++ b/crates/mozart/src/commands/install.rs
@@ -809,6 +809,7 @@ pub async fn execute(
         }
 
         let root_pkg = mozart_core::package::read_from_file(&composer_json_path)?;
+        root_pkg.validate_root_does_not_self_require()?;
         let missing = lock.get_missing_requirement_info(&root_pkg, dev_mode);
         if !missing.is_empty() {
             for line in &missing {
diff --git a/crates/mozart/src/commands/update.rs b/crates/mozart/src/commands/update.rs
index da2fd95..3a468d4 100644
--- a/crates/mozart/src/commands/update.rs
+++ b/crates/mozart/src/commands/update.rs
@@ -747,6 +747,7 @@ pub async fn execute(
         ));
     }
     let composer_json = package::read_from_file(&composer_json_path)?;
+    composer_json.validate_root_does_not_self_require()?;
     let composer_json_content = std::fs::read_to_string(&composer_json_path)?;
 
     let lock_path = working_dir.join("composer.lock");
diff --git a/crates/mozart/tests/installer.rs b/crates/mozart/tests/installer.rs
index f3eeabc..5d83bac 100644
--- a/crates/mozart/tests/installer.rs
+++ b/crates/mozart/tests/installer.rs
@@ -14,15 +14,40 @@ fn run_installer_fixture(ident: &str) {
     let mozart_bin: &Path = assert_cmd::cargo::cargo_bin!("mozart");
     let result = run_test(&parsed, mozart_bin)
         .unwrap_or_else(|e| panic!("failed to run {}: {:#}", path.display(), e));
-    let expected = parsed.expect_exit_code.unwrap_or(0);
-    assert_eq!(
-        result.exit_code,
-        expected,
-        "exit code mismatch for {}\n--- stdout ---\n{}\n--- stderr ---\n{}",
-        path.display(),
-        result.stdout,
-        result.stderr,
-    );
+
+    // Composer's `.test` format uses EXPECT-EXCEPTION to assert that the run
+    // throws an exception. PHP propagates uncaught exceptions as a non-zero
+    // exit; we don't yet match the exception class, but we do require Mozart
+    // to exit non-zero when the fixture expects an exception (and no explicit
+    // EXPECT-EXIT-CODE has been pinned).
+    if let Some(code) = parsed.expect_exit_code {
+        assert_eq!(
+            result.exit_code,
+            code,
+            "exit code mismatch for {}\n--- stdout ---\n{}\n--- stderr ---\n{}",
+            path.display(),
+            result.stdout,
+            result.stderr,
+        );
+    } else if parsed.expect_exception.is_some() {
+        assert_ne!(
+            result.exit_code,
+            0,
+            "expected non-zero exit (EXPECT-EXCEPTION) for {}\n--- stdout ---\n{}\n--- stderr ---\n{}",
+            path.display(),
+            result.stdout,
+            result.stderr,
+        );
+    } else {
+        assert_eq!(
+            result.exit_code,
+            0,
+            "exit code mismatch for {}\n--- stdout ---\n{}\n--- stderr ---\n{}",
+            path.display(),
+            result.stdout,
+            result.stderr,
+        );
+    }
 }
 
 macro_rules! installer_fixture {
@@ -281,10 +306,7 @@ installer_fixture!(
     install_security_advisory_matching_dependency,
     ignore = "mozart binary cannot yet run this fixture"
 );
-installer_fixture!(
-    install_self_from_root,
-    ignore = "mozart binary cannot yet run this fixture"
-);
+installer_fixture!(install_self_from_root);
 installer_fixture!(
     install_simple,
     ignore = "mozart binary cannot yet run this fixture"