Unify payment methods into single pay() function

Replace payLnurl(), payBolt11(), and payBolt12Offer() with a single pay()
method that auto-detects the destination type using bitcoin_payment_instructions.

Supported destinations:
- BOLT12 offers (lno...)
- LNURL (lnurl...)
- Lightning addresses (user@domain)
- Zero-amount BOLT11 invoices

This is a breaking change: BOLT11 invoices with pre-set amounts are no longer
supported through this API. The design enforces that the caller always
specifies the amount, which:
- Simplifies the API surface (one method instead of three)
- Ensures consistent amount handling across all payment types
- Protects against malicious LNURL services returning wrong amounts

The amount_msat parameter is now always required.

Author: martinsaposnic

index.d.ts

@@ -67,7 +67,17 @@ export declare class MdkNode {
     description: string,
     expirySecs: number,
   ): PaymentMetadata
-  payLnurl(lnurl: string, amountMsat: number, waitForPaymentSecs?: number | undefined | null): string
-  payBolt11(bolt11Invoice: string): string
-  payBolt12Offer(bolt12OfferString: string, amountMsat: number, waitForPaymentSecs?: number | undefined | null): string
+  /**
+   * Unified payment method that auto-detects the destination type.
+   *
+   * Only supports variable-amount destinations where we set the amount:
+   * - BOLT12 offers (lno...)
+   * - LNURL (lnurl...)
+   * - Lightning addresses (user@domain)
+   * - Zero-amount BOLT11 invoices
+   *
+   * BOLT11 invoices with embedded amounts are rejected - use a variable-amount
+   * destination instead. The amountMsat parameter is always required.
+   */
+  pay(destination: string, amountMsat: number, waitForPaymentSecs?: number | undefined | null): string
 }