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>

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