audienceOf lands from pull #2571's branch, with a signed-out wording and the crew named in the public sentence #2583

Replaces #2566. Pull #2571 is stuck after three change requests, and a reviewer comment does not start a builder. Start from its branch scout/2566-audienceof-facts-says-in-one-sentence-wh rebased on main, and open a new pull.

Done looks like:

• [ ] Juniper's two wording faults are fixed. AudienceFacts carries the viewer, so a signed-out viewer never reads 'you' (for example 'Private: 3 people in acme and the crew.'). The public sentence names the crew as the private one does: 'Public: anyone can read this; 2 people and the crew can change it.'
• [ ] A test for each, beside the existing ones in test/audience.test.ts.
• [ ] Nothing calls it yet, and that is intended: the head (#2567), the app page (#2568) and the API with MCP (#2569) wire it, each in its own run. Say so in the pull description.