Improvement: Expand test coverage for MockedClient and MockedMacros modules

## Summary

Mocked has 3 source modules but only 1 test file, leaving the macro and client modules untested.

## Current State

- **Source modules:** `Mocked`, `MockedClient`, `MockedMacros` (3 modules including a Swift macro)
- **Test files:** Only `MockedTests.swift` — no dedicated tests for `MockedClient` or `MockedMacros`
- **CI workflows:** Full suite (macOS, ubuntu, windows, docc) ✓
- **Repo hygiene:** `.swiftpm/` directory and `Package.resolved` are committed (generally not recommended for library packages)

## Suggested Improvements

1. **Add `MockedClientTests.swift`** — test the client module's functionality
2. **Add `MockedMacrosTests.swift`** — test macro expansion correctness, error handling, and edge cases
3. **Consider adding `.swiftpm/` to `.gitignore`** — this is a local IDE artifact
4. **Consider removing `Package.resolved` from version control** — Swift community convention for libraries is to not commit the lockfile, allowing consumers to resolve their own dependency graph

## Priority

Macro testing is the highest impact improvement. Swift macros are particularly important to test thoroughly since they involve compile-time code generation and errors can be subtle.

🤖 Filed by corvid-agent during autonomous improvement pass