refactor(server): inline client capability parsing

With capability negotiation in place, the consumer-side tolerance pattern is unnecessary. Drop the dead readDeclaredClientCapabilities helper, inline the parsing in ClientCapabilities.fromHello, delete a tautological assertion in the relay reconnect test, and document the compatibility rules in architecture.md.

packages/server/src/shared/client-capabilities.ts

@@ -3,21 +3,3 @@ export const CLIENT_CAPS = {
 } as const;
 
 export type ClientCapability = (typeof CLIENT_CAPS)[keyof typeof CLIENT_CAPS];
-
-const CLIENT_CAPABILITY_SET = new Set<string>(Object.values(CLIENT_CAPS));
-
-export function isClientCapability(value: string): value is ClientCapability {
-  return CLIENT_CAPABILITY_SET.has(value);
-}
-
-export function readDeclaredClientCapabilities(
-  capabilities: Record<string, unknown> | null | undefined,
-): ClientCapability[] {
-  if (!capabilities) {
-    return [];
-  }
-
-  return Object.entries(capabilities).flatMap(([key, value]) =>
-    value === true && isClientCapability(key) ? [key] : [],
-  );
-}