examples: add x402 usage-based settlement guide

examples/x402-upto-settlement.md

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+# x402 Usage-Based Settlement
+
+This example shows the **upto** (usage-based) settlement pattern
+for agent payments where the final amount is not known until the task completes.
+
+## How upto Settlement Works
+
+    1. Agent authorizes max amount  ->  service begins work
+    2. Service completes task        ->  settles actual amount
+    3. Remaining headroom released   ->  wallet balance updated
+
+| Pattern | Amount at auth | Amount settled |
+|---|---|---|
+| exact | Known upfront | Same as authorized |
+| upto  | Max cap authorized | Actual cost (<=max) |
+
+## Minimal Example
+
+    import { pay } from "@open-wallet-standard/core";
+
+    const result = await pay(
+      "agent-treasury",
+      "https://api.example.com/summarize",
+      "POST",
+      JSON.stringify({ text: "Long document..." })
+    );
+
+    if (result.payment) {
+      console.log("Settled :", result.payment.amount, result.payment.token);
+    }
+
+## Post-Settlement Accounting
+
+After a upto settlement, track three values:
+
+| Value | Description |
+|---|---|
+| Authorized | Max amount locked at request time |
+| Settled | Actual amount charged by service |
+| Released | Authorized minus settled (returned to balance) |
+
+    class UptoBillingTracker {
+      constructor() { this.entries = []; }
+
+      record(requestId, authorizedAmount, result) {
+        const settled = result.payment ? parseFloat(result.payment.amount) : 0;
+        const authorized = parseFloat(authorizedAmount);
+        const released = authorized - settled;
+        this.entries.push({ requestId, authorized, settled, released });
+        return { authorized, settled, released };
+      }
+
+      summary() {
+        return {
+          totalAuthorized : this.entries.reduce((s, e) => s + e.authorized, 0),
+          totalSettled    : this.entries.reduce((s, e) => s + e.settled, 0),
+          totalReleased   : this.entries.reduce((s, e) => s + e.released, 0),
+        };
+      }
+    }
+
+## OWS vs App Layer Boundary
+
+| Responsibility | OWS | Your App |
+|---|---|---|
+| Sign the payment authorization | yes | no |
+| Handle 402 -> retry flow | yes | no |
+| Track authorized vs settled amounts | no | yes |
+| Enforce per-session spending caps | no | yes |
+| Reconcile with external ledger | no | yes |
+
+OWS provides result.payment.amount (the settled amount).
+Everything above that is application-layer accounting.
+
+## Running This Example
+
+    npm install -g @open-wallet-standard/core
+    ows wallet create --name agent-treasury
+    node examples/x402-upto-settlement.js
+
+Get test USDC on Base Sepolia: https://faucet.circle.com