Fix per-destination deployment targets

[arhxam]

Summary

• Parse the documented per-platform deploymentTarget form for targets using supportedDestinations.
• Apply each version only to matching supported platforms while preserving the existing scalar behavior.
• Preserve the per-platform form when encoding project specs and cover unsupported-destination filtering.

Resolves #1577.

Test plan

• swift test (76 tests passed)
• Focused ProjectGenerator and SpecLoading suites
• Generated the issue-shaped iOS/macOS project with the built CLI and verified both deployment settings in project.pbxproj

AI assistance was used to investigate and draft this change. I reviewed the implementation and ran all tests and the end-to-end reproduction locally.