feat(provider): V4 type crate aimux-provider (phase 1 unit 1.6)

[cunninghamcard-bit]

Phase 1 unit 1.6: design draft of the V4 layers in RFC-0042 §3.1 / arcships#200:58. This adds a new aimux-provider crate holding only the AI SDK @ai-sdk/provider@4.0.21 language-model/v4 and shared/v4 types. It is a workspace member; nothing depends on it and nothing is wired to it.

• Dependencies: only serde and serde_json; no aimux crate (docs: full-chain AI SDK alignment design and impact map arcships/aimux#200 §2.1 constraint 1). cargo tree -p aimux-provider shows only those two, and cargo tree -i aimux-provider shows nothing depends on it. A test enforces this from the manifest.
• Wire: AI SDK JSON — camelCase fields, kebab-case type/role/sourceType tags, absent optionals omitted, bytes as base64 text.
• Layer rule:
  • prompt tool-call.input is parsed JSON (JsonValue);
  • output and stream tool-call.input is the provider's text (String);
  • stream text/reasoning deltas use delta;
  • no core invalid, error, retry or recording fields.
• Types:
  • Prompt and messages: user, assistant and tool parts, plus tool-result outputs.
  • Call options: tools, tool choice, response format, reasoning, nullable headers.
  • Output: Content, Source, ToolCall/ToolResult, FinishReason, Usage, GenerateResult.
  • Streaming: StreamPart (all 21 variants) and the stream result info.
  • Shared: FileData, Warning, ProviderOptions/Metadata.
• Not modelled: the LanguageModelV4 interface (doGenerate, doStream, supportedUrls) and abortSignal, which are behaviour rather than data.
• Tests: cargo test -p aimux-provider — exact JSON round trips for every variant, rejection of unknown tags, and the no-aimux-dependency check.

Note: this branch is cut from exp/unit2-ci so that CI triggers, so the diff against exp/restart also includes unit 2's ci.yml change.

Errors (D21)

aimux_provider::ProviderError is one enum with one variant per class in @ai-sdk/provider@4.0.21 src/errors/. It implements std::error::Error and Display:

• Display prints the upstream message, and the constructors fill in upstream's default messages;
• cause is reachable through source();
• name() returns the AI_* name;
• is_retryable() is true only for ApiCall, whose default mirrors upstream (status 408/409/429/≥500, and an explicit value wins).

There is no serde and no wire format; mapping to aimux error codes and golden fixtures is deferred to 3.3.

variant	upstream class (src/errors/…)
AiSdk	AISDKError (ai-sdk-error.ts)
ApiCall	APICallError (api-call-error.ts)
EmptyResponseBody	EmptyResponseBodyError (empty-response-body-error.ts)
EvaluationUnsupportedQuestionType	EvaluationUnsupportedQuestionTypeError (evaluation-unsupported-question-type-error.ts)
InvalidArgument	InvalidArgumentError (invalid-argument-error.ts)
InvalidPrompt	InvalidPromptError (invalid-prompt-error.ts)
InvalidResponseData	InvalidResponseDataError (invalid-response-data-error.ts)
JsonParse	JSONParseError (json-parse-error.ts)
LoadApiKey	LoadAPIKeyError (load-api-key-error.ts)
LoadSetting	LoadSettingError (load-setting-error.ts)
NoContentGenerated	NoContentGeneratedError (no-content-generated-error.ts)
NoSuchModel	NoSuchModelError (no-such-model-error.ts)
NoSuchProviderReference	NoSuchProviderReferenceError (no-such-provider-reference-error.ts)
TooManyEmbeddingValuesForCall	TooManyEmbeddingValuesForCallError (too-many-embedding-values-for-call-error.ts)
TypeValidation	TypeValidationError (type-validation-error.ts, incl. TypeValidationContext)
UnsupportedFunctionality	UnsupportedFunctionalityError (unsupported-functionality-error.ts)

get-error-message.ts is a helper, not a class; it is used for the cause text inside the messages. tests/errors_test.rs (7 tests) covers construction, Display text, is_retryable and the source chain.