discovery: generate capability JSON from the registry

A capability is a descriptor plus what the endpoint declares (device/Capability.ts), and its discovery object is
generated from the two. The new API is bridge.addDevice({ endpointId, name, categories, ... }) and
device.add(PowerController, options): both throw a DeclarationError that names the endpoint, the interface and
the instance (device/validate.ts), and leave the bridge and the device as they were. AlexaInterface is the same
Capability with the 1.x methods on it; it, ActionMapping and the enums moved to src/compat/, Device to src/device/.

What a 1.x caller can observe:
- every endpoint ends with { type: "AlexaInterface", interface: "Alexa", version: "3" } (alexa-interface.html);
  new Alex2MQTT(..., { alexaInterface: false }) leaves it out
- the fields of a capability object come in the order of Amazon's examples; their content is unchanged
- addCapability() with a name that is not an interface throws (1.5.2 announced it with the version "UNKNOWN")
- ActionMapping takes the payload as an object; a JSON string is parsed (1.5.2 sent the string), any other throws
- what Alexa would reject in a 1.x declaration is not refused: device.check() lists it and the bridge logs each
  line once, as "warning: ..." through the log hook, when it answers a discovery
- a device whose JSON cannot be built is left out of the answer and reported as an error event
- PowerController and EndpointHealth are the descriptors and keep ON/OFF and OK/UNREACHABLE; PowerState is new

Tests: six zoo devices declared the 1.x way give the JSON that Alexa accepted from 1.5.2 on 2026-09-28, plus the
Alexa capability. npm test: 85 pass (was 57) in 10-12 s, also on Node 18.20.8 and 20.20.2.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/registry/schema.ts

@@ -65,11 +65,13 @@ function shown(input: unknown): string {
   return text.length > 60 ? `${text.slice(0, 57)}...` : text;
 }
 
-function mismatch(path: string, expects: string, input: unknown): SchemaError {
+/** The error of a value that is not what a schema expects. */
+export function mismatch(path: string, expects: string, input: unknown): SchemaError {
   return new SchemaError(path, `expected ${expects}, got ${shown(input)}`);
 }
 
-const at = (path: string, key: string): string => (path ? `${path}.${key}` : key);
+/** The path of a key of the object at path. */
+export const at = (path: string, key: string): string => (path ? `${path}.${key}` : key);
 
 function isRecord(input: unknown): input is Record<string, unknown> {
   return typeof input === "object" && input !== null && !Array.isArray(input);
@@ -117,7 +119,12 @@ function literal<V extends string | number | boolean | null>(value: V): Schema<V
 }
 
 function enumeration<V extends readonly string[]>(...values: V): EnumSchema<V[number]> {
-  return { ...schema<V[number]>(values.join(" | "), (input) => values.includes(input as string)), values };
+  return oneOf(values, values.join(" | "));
+}
+
+/** One of a list too long to print in an error message: expects says what the list is. */
+function oneOf<V extends string>(values: readonly V[], expects: string): EnumSchema<V> {
+  return { ...schema<V>(expects, (input) => values.includes(input as V)), values };
 }
 
 function unknown(): Schema<unknown> {
@@ -229,6 +236,7 @@ export const s = {
   boolean,
   literal,
   enum: enumeration,
+  oneOf,
   unknown,
   optional,
   nullable,