Give decision API rejections a stable error code #114

Decision API rejections are plain Errors with no status code, so llamadart has to classify them by message text (leehack/llamadart#620, webgpu_decision.dart _bridgeException): "Load the decision head again", "No model loaded", "Bridge has been disposed", "was cancelled", "during active generation" and "decision encoder context". Rewording any of these silently changes which exception llamadart throws.

Proposal: before the decision API's first published assets, attach a stable code to rejected errors: the core status (for example -4) or a string such as 'head-lost', 'no-model', 'disposed', 'cancelled', 'busy', 'context'. Document the codes in docs/api.md next to apiVersion 1.