feat(plugin-market): add the Custom plugin markets bundle and Discover tab

Read configured plugin market indexes on the Host, search them with fuzzy
multilingual matching, and install a chosen version from the Plugins page.

- dsh-plugin-market-index validates format-1 indexes and builds install
  arguments; inline icons are capped at 32 KiB.
- The pluginMarket service fetches configured http(s) URLs, parses bodies in
  a Worker Thread, caches documents, and pages filtered, ordered results with
  installation status and engines.dsh compatibility.
- Discover is an opt-in experimental bundle; the Plugins page gains a
  plugins.tab slot and confirmed installs.
- The plugin_market tool lists indexes, plugins, and versions for Creator mode.
- Plugin compatibility reads engines.dsh before DSH peer dependencies.

packages/experimental/README.md

@@ -58,6 +58,7 @@ Promotion requires a named product role, documented public behavior and limitati
 | [`speech-to-text-sensevoice`](speech-to-text-sensevoice/README.md) | Managed local SenseVoice inference | — |
 | [`api-speech-to-text`](api-speech-to-text/README.md) | Authenticated transient transcription Remote | `ctx.speechController` |
 | [`client-ui-voice-input`](client-ui-voice-input/README.md) | Microphone capture and guarded draft insertion | — |
+| [`plugin-market-bundle`](plugin-market-bundle/README.md) | Default-disabled optional custom plugin markets: the market service and the Discover tab | — |
 | [`voice-input-bundle`](voice-input-bundle/README.md) | Default-disabled optional voice input composition | — |
 | [`agent-team-profile`](agent-team-profile/README.md) | Agent Teams collaboration, tools, and Web UI bundle | — |
 | [`agent-team`](agent-team/README.md) | Named teammates with direct messages and a durable shared task board | `ctx.agentTeams` |