A back-and-forth conversation in a single link. You record a 4-bar call. They open it, hear it loop, and add their response in the second grid. The final URL plays both phrases as one back-and-forth — a tiny musical exchange that lives only in the link.
The whole conversation is 18 bytes of URL: 8 bytes for your call, 8 bytes for their answer, 1 byte for BPM, 1 reserved. That's ?p= in 24 base64url characters. Add a ?from=abcd-efgh chain so each side knows who's speaking, and the link is complete.
Three modes, decided by what's in the URL:
Empty → record your call. Copy the link, paste in iMessage.
Call only (response half is all zeros) → you see their call, hear it loop, add your answer in the second grid. Send back.
Conversation (both halves filled) → play loops call → response → call → response as one back-and-forth phrase.
The ↓ they answer ↓ bridge between the grids isn't just decoration — it's the audible boundary in playback, the moment where the "voice" switches. That space is what makes the two phrases read as a conversation rather than 8 random bars.
