[TASK] Base GitHub Actions build workflow

[kirich1409]

Context

.github/workflows/ci.yml runs on pull_request and push main: macos-14 runner, latest Xcode, swift build && swift test.

Acceptance criteria

• Workflow green on clean main.
• Fails if swift build fails.
• Caches .build and ~/Library/Caches/org.swift.swiftpm between runs.

Files to touch (estimate)

• .github/workflows/ci.yml

Implementation notes

n/a

Dependencies

• Blocked by [TASK] Create Package.swift with 4 targets #19

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved