dsh-matrix-terminal-balance: Connect DeepSeek account balance to DSH plugin interface

`dsh-matrix-terminal-balance` is a host-side plugin for `dsh-matrix-terminal`, maintained by victorcastro55, used for classification model inference. It provides the plugin interface with DeepSeek account balance via an authenticated route: upon request, it parses the DEEPSEEK_API_KEY using the credential service, calls the platform balance interface (with an 8-second timeout), caches successful responses for 30 seconds, and merges concurrent in-flight requests. On failure, it returns `ok:false` with a `reason` (`no-key`, `network`, `http-<status>`, `protocol`) instead of allowing the route to error out. The route utilizes the host's `/api` authentication channel; unauthenticated requests return 401, and requests from external Origins return 403. The key remains within the host process, and the browser receives only a decimal string. The installation command is `dsh plugin --profile web add github:victorcastro55/dsh-matrix-terminal-balance`, requiring DSH 0.2.1-rc.5, the connection and credentials services, and a configured DeepSeek key. The plugin has not yet been published to npm.