Add four lights technical poem

[DYSfu]

/claim #76

Summary

• Adds POEM.md at the repository root with an original five-stanza technical poem.
• Includes short demo proof at demos/four-lights-poem-demo.mp4.

One-line note: I used a four-lights workflow image for scope, messages, escrow, evidence, and payout because it matches the platform's trust path without reusing the common ledger or harbor metaphors.

Validation

• Stanza count check: POEM.md: 5 stanzas
• ffprobe demos/four-lights-poem-demo.mp4
• git diff --check