chore(release): cut v0.21.1 as the stable adapter for DSH 0.1.7-rc.1

The rc.1 adapter first went out as 0.21.0-rc.1, but DSH 0.1.7-rc.1 is
upstream's release batch, so the plugin that targets it must not sit on a
prerelease channel. Re-cut it as v0.21.1: the version is now free of `-`,
so release.yml publishes it under npm's `latest` instead of `alpha`.

Version literals only — package.json, dsh.plugin.json and
SIDEBAR_SERVICE_VERSION (tests/service.spec.ts still pins the last two
together). No code change: the peer floor stays ^0.1.7-rc.1 and the
support line stays "DSH 0.1.7-rc.1 and later".

Docs follow the channel move: README/README_EN/guide drop the "alpha
channel" framing, the install command becomes @latest, and every older
DSH line now gets an explicitly pinned command in the version table
(@0.19.1 and so on) because `latest` no longer covers them. The rc.1
design record keeps its history and gains a §8 for the re-cut.

src/client/service.ts

@@ -633,7 +633,7 @@ export function matchUrlTarget(tabs: readonly TabDescriptor[], url: URL): TabDes
  * The plugin version this service instance reports. Keep in lockstep with
  * `package.json`'s version — `tests/service.spec.ts` asserts the pair.
  */
-export const SIDEBAR_SERVICE_VERSION = '0.21.0-rc.1'
+export const SIDEBAR_SERVICE_VERSION = '0.21.1'
 
 /**
  * Monotonic capability list consumers use to gate new API usage (features