Private-only doorbell: drop the public fallback, surface failures → 0.2.0-next.26

[HomemadeToast57]

Supersedes the compat approach in #21. Nothing real depends on the pre-private doorbell, so the fallback is deleted rather than carried.

Why

The public compat channel cost a second realtime.messages insert on every row change, forever, left an anon-joinable topic protected only by the obscurity of a schema name that ships in every bundle, and had no retirement mechanism. A permanent tax and a permanent leak surface, to serve users who don't exist.

What changes

• Every doorbell topic is wristband-gated. A socket with no gateway-minted token hears nothing at all.
• One broadcast per row change instead of two.
• The id-stripping workaround from next.25 is gone with the channel that needed it (Realtime's injected message-uuid was only ever observable there).
• Failure is surfaced, not disguised. With no degraded path to slink onto: capped-backoff retry (1s → 5s → 15s → 60s, reset by a good join) plus doorbell.status() / onStatus reporting connecting | live | unauthorized | unavailable. HTTP reads keep working, so an app is never broken — just not live, visibly.
• mint() returns a discriminated MintResult so a 403 ("you may not watch this app") is distinguishable from a network failure or 404/503. This matters most for public apps, where every visitor is normally admitted and must never be told they're unauthorized because a request dropped.

Public no-auth apps — the primary path

Verified on dev against the real gateway route:

anonymous mint: 200
  topics: {"app":"bool:app_…:app","user":null}   ← nobody signed in
  join: SUBSCRIBED
anonymous viewer got the ROW: true
eavesdropper (anon key, no wristband) on old public topic: heard 0

"Private" means wristband-gated, not login-gated — for a public app the gateway hands one to every visitor.

154 pass, typecheck clean. Pairs with codehs/bool#570.