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>

1 files changed, 1 insertions, 0 deletions

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");