[longzhi]

Part 2 of #148, stacked on #149: review that one first. Once #149 is merged, this PR's base becomes main.

What changes

• Daemon (daemon/src/activity.rs, main.rs):
  • Every item in tasks gets an id: eight hex digits of FNV-1a over the activity's key. It stays the same for as long as the activity lasts.
  • A focus event from the box names a card; the daemon opens that activity's source through source_opener::open, the same path K2 uses.
  • If the activity has ended, or has no source, the daemon logs it and does nothing. It does not fall back to K2's choice.
• Firmware:
  • Each card keeps the id it came with. Only short alphanumeric ids are accepted, so the id can go back out in a JSON line as is.
  • A tap on a card sends {"version":1,"event":"focus","task":"<id>"}.
  • Cards from an older daemon have no id, and a tap on one sends nothing.
• Docs: protocol.md (the id field and the focus event), device-menu.md, and both READMEs.

One side effect

Two cards with the same title that swap places now send the box a redraw, because their ids moved. Before, the two snapshots were identical and deduplicated. The box needs the new order, or a tap could open the wrong agent. The CI test parallel_workflows_each_report_their_own_end now expects that task.start. task.start is silent. A redrawn needs-input screen keeps suppress_audio as before.

Mac and Linux

Opening a source lives in the shared daemon, so a card tap works wherever K2 works, on macOS and Linux alike. Neither App changes.

Testing

• cargo test --workspace passes.
• New tests:
  • Daemon: a_tapped_card_leads_to_its_own_activity (the id, the card's own source rather than K2's choice, and an unknown id) and a_focus_event_names_the_tapped_card.
  • Firmware: a_tapped_card_asks_for_its_agent and a_card_from_an_older_daemon_or_with_an_odd_id_names_nothing.
• On box 30:ED:A0:A4:0D:08 (2026-10-15): with three cards sent over serial carrying ids, taps on the top, middle and bottom card sent focus with aaaa0001, bbbb0002 and cccc0003 in turn.
• End to end on 2026-10-21, with this branch's daemon and firmware on box 30:ED: a tap on the top card, then CX:EROS-TRAINING-INFRA, logged opened a tapped card's source link=codex://threads/…, and Codex came to the front on that thread.

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