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/lazy-module-load.test.ts

@@ -66,7 +66,7 @@ describe("lazy provider module loading", () => {
 		expect(result.loadedSpecifiers).toEqual([]);
 	});
 
-	it("loads only the Anthropic SDK when calling the root lazy wrapper", () => {
+	it("loads only the Anthropic SDK when streaming through the lazy API wrapper", () => {
 		const result = runProbe(`
 			const model = {
 				id: "claude-sonnet-4-6",
@@ -81,7 +81,7 @@ describe("lazy provider module loading", () => {
 				maxTokens: 8192,
 			};
 			const context = { messages: [{ role: "user", content: "hi" }] };
-			await mod.streamSimpleAnthropic(model, context).result();
+			await mod.anthropicMessagesApi().streamSimple(model, context).result();
 		`);
 
 		expect(result.loadedSpecifiers).toEqual(["@anthropic-ai/sdk"]);