Add integration tests for PreviewSession.compile() with BuildContext

[obj-p]

PreviewSession.compile() has Tier 1/Tier 2 branching logic based on BuildContext that is only tested via the MCP agent integration test, not in the Swift test suite.

Add tests for:

• compile() with Tier 1 BuildContext (bridge-only, @testable import)
• compile() with Tier 2 BuildContext (source compilation, ThunkGenerator)
• tryLiteralUpdate() returning nil for Tier 1

These need a real SPM project to build against (like examples/spm/), so they'll be slow (~5-10s). Consider a separate test suite or --filter convention for slow tests.

Identified in PR #3 review.

[obj-p]

Update: PR #21 added agent-level integration test skills that cover the full MCP flow including compile() with both tiers. However, the Swift test suite tests (swift test) for PreviewSession.compile() with Tier 1/Tier 2 BuildContext that this issue specifically requests are still missing.