feat(ai): move API implementations to src/api with lazy wrappers (phase 2)

Stream implementations move from src/providers/ to src/api/, renamed by
API id (anthropic.ts -> anthropic-messages.ts, google.ts ->
google-generative-ai.ts, mistral.ts -> mistral-conversations.ts,
amazon-bedrock.ts -> bedrock-converse-stream.ts). Every module now
exports exactly stream/streamSimple; shared helpers move alongside.

New ProviderStreams dispatch contract in types.ts, lazyApi() wrapper in
api/lazy.ts, and one .lazy.ts wrapper per API. Bedrock's wrapper keeps
the node-only variable-specifier import and setBedrockProviderModule()
(now taking ProviderStreams).

providers/register-builtins.ts deleted; interim until the compat
entrypoint lands, builtin api-registry registration lives in stream.ts
and lazy wrappers are exported from the root barrel. Old per-API lazy
exports (streamAnthropic, ...) are gone; package.json subpaths retarget
to dist/api/.

packages/ai/test/scratch.ts

@@ -2,10 +2,12 @@
 // Run from packages/ai: node test/scratch.ts
 // Requires ANTHROPIC_API_KEY.
 
+import { anthropicMessagesApi } from "../src/api/anthropic-messages.lazy.ts";
 import { createModels, getModels, type Provider } from "../src/models.ts";
-import { streamAnthropic, streamSimpleAnthropic } from "../src/providers/register-builtins.ts";
 import type { Context } from "../src/types.ts";
 
+const anthropicApi = anthropicMessagesApi();
+
 // ---------------------------------------------------------------------------
 // 1. Define a provider. In the final design this comes from
 //    `@earendil-works/pi-ai/providers/anthropic` as `anthropicProvider()`;
@@ -33,8 +35,8 @@ const anthropic: Provider<"anthropic-messages"> = {
 	getModels: async () => getModels("anthropic"),
 
 	// shared lazy API implementation (loads the SDK on first request)
-	stream: streamAnthropic,
-	streamSimple: streamSimpleAnthropic,
+	stream: anthropicApi.stream,
+	streamSimple: anthropicApi.streamSimple,
 };
 
 // ---------------------------------------------------------------------------