test: add materialized-workspace contract tests for catalog combinations

[lloydrichards]

Context

Catalog validation proves that definitions and dependencies are valid, but some failures appear only after modules are combined and applied. Existing generated-workspace tests cover important journeys, but they are slower and depend on host directories, subprocesses, and external tools.

Benefits

An in-memory contract-test harness would let us verify the repository the catalog claims to generate while still exercising the production Blueprint, Plan, composition, and Apply behavior.

Failures could report the selection, starting fixture, changed paths, and relevant contents without leaving temporary repositories behind.

Expected changes

• Add a reusable scaffold integration-test helper that starts from a repository fixture and materializes a selection in isolation.
• Cover representative module combinations rather than attempting every possible combination.
• Include incremental additions, preservation of user-owned content, and deterministic generation.
• Test idempotency by rebuilding the Plan from the first resulting workspace before applying again.
• Retain host-filesystem E2E tests for Git, package managers, subprocesses, and platform behavior.

This complements #181's real-filesystem lifecycle contract test. This issue owns broad, fast coverage of representative built-in catalog combinations and their final repository trees; it does not replace the host-backed vertical test or own general workflow orchestration.

The harness should live with scaffold integration testing. The catalog package should remain typed, deterministic data rather than gaining a filesystem dependency. Contributed catalog fragments can be added only if #249 establishes how they compose.

Done when

• Tests materialize a multi-module create journey without host filesystem writes.
• Tests cover at least one incremental-add journey.
• Replanning and applying an unchanged generated workspace has an explicit expected result.
• Unexpected final file changes produce a useful test failure.
• The responsibilities retained by host-filesystem E2E tests are documented.

Depends on

• refactor: preview generation around a reusable in-memory workspace #250

Related

• End-to-end planning fixtures and ordering tests #31
• Virtual File Tree / In-Memory Generation #108
• Separate scaffold workflow orchestration from CLI interaction #181
• Proposal: establish the Community Catalog foundation #249

[lloydrichards]

A concrete combination for this harness is the import-validation client module plus client-react-http-api. Both contribute to the same components slot:

<ImportValidationCard />
<RestCard />

Assert that the materialized app.tsx retains both contributions. Independent whole-file snapshots would lose this intent; the existing composers must still run.

Also cover an existing user field, a skipped conflict, and an ancestor obstruction. Rebuild Plan before the second Apply when checking idempotency. This tests the shared workspace contract from #250 while leaving real package-manager and host behavior to E2E tests.