Follow-Up to #13 & #17: Add copy feedback and source attribution

[alvst]

Summary

Completes copy feedback and source attribution without weakening the modern reliable paste handoff.

• Shows a short non-key copy toast only after a successful explicit Copy.
• Preserves the existing successful-copy history promotion.
• Writes the org.nspasteboard.source provenance marker for every successful Pesty copy path, including plain-text, image, file, rich-text, color, and link content.
• The monitor skips Pesty-marked writes to avoid duplicate/self-attribution and still respects external source markers.
• Invalid payloads do not trigger a stale paste or copy confirmation.

Dependency

This draft is based on #55 and should merge after it. It will be rebased onto main once its dependencies land.

Validation

• swift build
• swift test — 13 tests passed
• git diff --check

Screenshot

Any visual QA for this PR uses Debug Demo only (Pesty --demo).