feat!: move to the alpha track — DSH 0.1.2-alpha.x only (v0.18.0-alpha.0)

DSH itself is still on its 0.1.2-alpha line (npm `alpha` dist-tag), so the
plugin mirrors it: v0.18.0-alpha.0 publishes under the npm `alpha`
dist-tag and supports DSH 0.1.2-alpha.x only. Stable-DSH (0.1.0-rc.8 ~
0.1.1-rc.2) users stay on v0.17.1 (npm `latest`). Verified end-to-end
against a real 0.1.2-alpha.2 host: full vitest suite, typecheck,
build/pack, consumer type surface, and the headless mount smoke 14/14
(the prerelease version also passes `dsh plugin add file:`).

- peer floor raised `^0.1.0-rc.8` -> `^0.1.2-alpha.2` (the mount install
  no longer warns: the old floor could not semver-match a prerelease
  host); dsh.plugin.json engines.dsh `>=0.1.2-alpha.2`.
- Dropped the pre-alpha compatibility layer:
  - e2e host RPC collapsed from the dot/slash dual dialect to slash-only
    (rpcAttempt; the 404-probe negotiation and its cache are gone), and
    parseLaunchUrl now requires the ?token= launch URL;
  - MarkdownText labels collapsed to the nested single shape (the cast
    became unnecessary against the alpha.2 declarations);
  - the defunct `@deepseek-ai/dsh-client-runtime` row is gone from the
    chunk externals allowlist (chunk-loader / tsdown / two specs) and
    from dsh.client.inject.
- market-manifest guard now allows an exact prerelease version (alpha
  cuts are not the market's verified `latest` target); release.yml picks
  the npm dist-tag from the version (prerelease -> `alpha`, else
  `latest`) so an alpha can never hijack `latest`.
- Docs retargeted: AGENTS.md §2/§3, README/README_EN badges + v0.18.0
  -alpha.0 section + install note (`dsh-better-sidebar@alpha`), guide.

src/client/chunk-loader.ts

@@ -62,18 +62,11 @@ type ChunkFactory = (require: (spec: string) => unknown) => ChunkExports
  * CLIENT_EXTERNALS in tsdown.config.ts — the chunk builds keep these
  * external and the loader resolves them here). A superset is safe: the
  * require only answers what the chunk actually asks for. The shell's static
- * module table seeds React, Cordis, and the UI libraries (primitives/slots);
- * `dsh-client-runtime/client` normalizes onto the runtime package row
- * (stripClientSuffix). dsh-client-web-react / dsh-client-schema-form were
- * dropped in DSH 0.1.0-rc.8 (no rc.8 publish, nothing requires them) — the
- * chunks never asked for them, so they no longer belong here.
- *
- * DSH 0.1.2-alpha.1 removed the `dsh-client-runtime` package outright (the
- * seed table gained bare-name `@deepseek-ai/dsh-client-store` instead); the
- * runtime/client row below stays for 0.1.1-rc.x hosts — no chunk requires
- * it, and {@link buildExternalsRequire} keeps an unresolvable spec
- * undefined until a chunk actually asks (only then is it a loud error), so
- * the entry is inert on 0.1.2-alpha.1+.
+ * module table seeds React, Cordis, and the UI libraries (primitives/slots).
+ * `@deepseek-ai/dsh-client-runtime` was removed upstream in DSH 0.1.2-alpha
+ * (its seed row became bare-name `@deepseek-ai/dsh-client-store`) and no
+ * chunk ever required it, so its row is gone; so are dsh-client-web-react /
+ * dsh-client-schema-form, dropped back in DSH 0.1.0-rc.8.
  */
 export const CHUNK_EXTERNALS: readonly string[] = [
   'react',
@@ -83,7 +76,6 @@ export const CHUNK_EXTERNALS: readonly string[] = [
   'cordis',
   '@deepseek-ai/dsh-client-ui-slots',
   '@deepseek-ai/dsh-client-ui-primitives',
-  '@deepseek-ai/dsh-client-runtime/client',
 ]
 
 /** Chunk script endpoint served by the plugin host half (src/bundle-route.ts). */