Add generated-package round-trip validation fixtures

## Problem statement Compiled packages must survive the same distribution paths as hand-authored packages. Existing package export/import validation proves structural round trips, but compiled packages need fixtures that assert behavior-critical files and metadata survive unchanged. ## Scope - Add fixture workflows compiled from representative source workspaces. - Export and import generated packages using the existing package commands. - Verify manifests, workflows, skills, references, setup declarations, capabilities, and content digests after round trip. - Include failure fixtures for malformed generated packages. ## Acceptance criteria - A compiled package round-trips through export/import and still passes `lineage package validate` from #31. - Behavior-critical generated artifacts are preserved exactly or with documented deterministic normalization. - Tests fail when workflow steps, references, setup requirements, or capabilities disappear. - Fixtures are small enough to maintain and documented for future compiler changes. ## Dependencies - Builds on #31 package validation and the existing export/import path. - Depends on artifact generation and compile-time consistency checks. ## Non-goals - No registry publish/pull requirement in the first version. - No live provider execution. - No broad fixture matrix for every future provider. ## Sequencing Land after compile-time consistency checks and before provider materialization diagnostics.