[TASK] Create Package.swift with 4 targets

[kirich1409]

Context

Author Package.swift with targets DevsweepCore, DevsweepCorePrivate, DevsweepModules, and devsweep executable. Dependency direction: devsweep → DevsweepCorePrivate → DevsweepCore and devsweep → DevsweepModules → DevsweepCore. Modules must NOT reference DevsweepCorePrivate.

Acceptance criteria

• swift build succeeds with empty placeholder targets.
• DevsweepModules does NOT list DevsweepCorePrivate in its dependencies.
• devsweep executable lists DevsweepCore, DevsweepCorePrivate, DevsweepModules, and swift-argument-parser.

Files to touch (estimate)

• Package.swift
• Sources/DevsweepCore/Placeholder.swift
• Sources/DevsweepCorePrivate/Placeholder.swift
• Sources/DevsweepModules/Placeholder.swift
• Sources/devsweep/main.swift

Implementation notes

• Require Swift 5.9+ for internal import (SE-0409).
• Set platforms: [.macOS(.v14)].

Dependencies

None.

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