feat(printer): USB transport behind the ESC/POS render layer

The ESC/POS printer drivers were TCP-only — every path went through
sendRaw/probe to a raw socket on port 9100. Add a USB transport behind
the existing render layer without touching a single render*() function.

- printer-escpos.ts: sendRawUsb/probeUsb write the same ESC/POS bytes to a
  kernel usblp char device (/dev/usb/lp0) via a plain fs write — no
  libusb/CUPS/native dep (keeps MIT-only + minimal-deps appliance). A
  discriminated Transport + transportFromConfig/sendTo/probeTo dispatch the
  wire; anything not transport:"usb" is TCP, so existing host-only configs
  need no migration. Shared transportField/devicePathField config fields.
- cashino + rongta resolve a Transport once; both are reachability-only over
  USB, and the Rongta's HTTP status page degrades to the open-the-node probe
  over USB (no guessed paper/cover — the standing honesty rule). host/port
  made not-required so a USB printer needs neither.
- Tests: printer-escpos.test.ts (USB writes the exact rendered bytes; probe
  present/absent; transportFromConfig TCP back-compat) + printer-cashino.test.ts
  (USB-configured driver prints to the node, ready/offline).

USB itself is unverified on hardware (the on-site printers are networked);
the appliance-side usblp + udev provisioning is tracked as open-questions #14.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

packages/devices/src/drivers/printer-escpos.test.ts

@@ -1,9 +1,15 @@
-import { describe, expect, it } from "vitest";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import {
   renderTicket,
   renderReceipt,
   renderWindowChargeNotice,
   renderSubscriptionCard,
+  probeUsb,
+  sendRawUsb,
+  transportFromConfig,
   stamp,
 } from "./printer-escpos.js";
 
@@ -103,6 +109,69 @@ describe("CP852 character mapping (the misprint fixes)", () => {
   });
 });
 
+describe("USB transport (sendRawUsb / probeUsb / transportFromConfig)", () => {
+  // A regular file stands in for the usblp character device: open(O_WRONLY) + write
+  // is the same syscall path. This proves the transport is byte-blind — the EXACT
+  // ESC/POS stream renderTicket produces lands at the device path, with no transport
+  // touching a rendered byte (the whole point of the seam).
+  let dir: string;
+  let devicePath: string;
+
+  beforeEach(() => {
+    dir = mkdtempSync(join(tmpdir(), "escpos-usb-"));
+    devicePath = join(dir, "lp0");
+    // A real usblp node already EXISTS (created by the kernel on enumeration); we open
+    // it O_WRONLY without O_CREAT, never create it. Pre-create the stand-in file so the
+    // test mirrors that — opening an ABSENT path means "printer not present" (offline).
+    writeFileSync(devicePath, "");
+  });
+  afterEach(() => {
+    rmSync(dir, { recursive: true, force: true });
+  });
+
+  it("writes the exact rendered ESC/POS bytes to the device path", async () => {
+    const payload = renderTicket({ ticketId: "12345678901", issuedAt: "2026-06-21T10:00:00.000Z" });
+    await sendRawUsb(devicePath, payload, 1000);
+    const written = readFileSync(devicePath);
+    expect(written.equals(payload)).toBe(true);
+  });
+
+  it("rejects when the device path can't be opened (printer not present)", async () => {
+    await expect(
+      sendRawUsb(join(dir, "absent-lp0"), Buffer.from([0x1b, 0x40]), 1000),
+    ).rejects.toThrow();
+  });
+
+  it("probeUsb resolves for an existing node, rejects for a missing one", async () => {
+    await expect(probeUsb(devicePath, 1000)).resolves.toBeUndefined();
+    await expect(probeUsb(join(dir, "nope"), 1000)).rejects.toThrow();
+  });
+
+  it("transportFromConfig: transport=usb selects the char device (default /dev/usb/lp0)", () => {
+    expect(transportFromConfig({ transport: "usb", devicePath: "/dev/usb/lp1" })).toEqual({
+      kind: "usb",
+      devicePath: "/dev/usb/lp1",
+    });
+    expect(transportFromConfig({ transport: "usb" })).toEqual({
+      kind: "usb",
+      devicePath: "/dev/usb/lp0",
+    });
+  });
+
+  it("transportFromConfig: anything else is TCP (back-compat with host-only configs)", () => {
+    expect(transportFromConfig({ host: "10.0.0.9" })).toEqual({
+      kind: "tcp",
+      host: "10.0.0.9",
+      port: 9100,
+    });
+    expect(transportFromConfig({ host: "10.0.0.9", port: 9101 })).toEqual({
+      kind: "tcp",
+      host: "10.0.0.9",
+      port: 9101,
+    });
+  });
+});
+
 describe("stamp (Albanian date format)", () => {
   it("formats an ISO time as '<day> <Month> <year> HH:MM:SS'", () => {
     // Local-time dependent, so assert the structure + the Albanian month name.