feat(ai): compat entrypoint, core-only root barrel (phase 5)

The root barrel is now core-only and side-effect free: types,
createModels/createProvider, auth substrate, lazyStream/lazyApi, faux,
utils. Generated catalogs, api-registry, env-api-keys, images, global
stream functions, and per-API lazy wrappers leave the root.

New @earendil-works/pi-ai/compat preserves the old surface verbatim as
a strict superset of the root: api-dispatch stream/complete with env
key injection, the builtin registration side effect (skip-if-present so
it cannot clobber earlier overrides), deprecated getModel/getModels/
getProviders aliases of the new getBuiltin* reads in providers/all,
lazy api wrappers + setBedrockProviderModule, and image generation.
Compat dies with the coding-agent ModelManager migration.

Packaging: exports map gains ./compat, ./providers/*, ./api/*;
sideEffects array lists only the effectful modules.

Old-global imports across agent/coding-agent/examples and pi-ai tests
switch to /compat (path-only; compat is a superset). The coding-agent
extension loader resolves the pi-ai ROOT specifier to compat, so
existing user extensions using the old global API keep working at
runtime until compat is removed. vitest configs alias /compat to src;
browser smoke imports old globals from /compat.

packages/ai/package.json

@@ -5,11 +5,28 @@
 	"type": "module",
 	"main": "./dist/index.js",
 	"types": "./dist/index.d.ts",
+	"sideEffects": [
+		"./dist/compat.js",
+		"./dist/images.js",
+		"./dist/providers/images/register-builtins.js"
+	],
 	"exports": {
 		".": {
 			"types": "./dist/index.d.ts",
 			"import": "./dist/index.js"
 		},
+		"./compat": {
+			"types": "./dist/compat.d.ts",
+			"import": "./dist/compat.js"
+		},
+		"./providers/*": {
+			"types": "./dist/providers/*.d.ts",
+			"import": "./dist/providers/*.js"
+		},
+		"./api/*": {
+			"types": "./dist/api/*.d.ts",
+			"import": "./dist/api/*.js"
+		},
 		"./anthropic": {
 			"types": "./dist/api/anthropic-messages.d.ts",
 			"import": "./dist/api/anthropic-messages.js"